Minor issues with preprocessed TypeScript

[nstuyvesant]

Describe the design limitations

1. svelte-preprocess does not remove lang="ts" from generated files - only lang="typescript". This differs from vitePreprocess as it expects lang="ts" and is confusing for users who might need svelte-preprocess after starting with vitePreprocess.
2. Generated svelte components do not have a line break following the opening <script> tag.
3. Generated svelte components do not have the generics attribute removed from <script> tags.
4. Generated svelte components do not apply prettier configuration. While we can manually run prettier on the dist folder but it would be nice if svelte-preprocess followed the configured rules.

To Reproduce

1. Create a Svelte component with the following code:

<script lang="typescript" generics="T extends BaseChartOptions">
	import { onMount, afterUpdate, onDestroy, createEventDispatcher } from 'svelte'
	import type { Charts, ChartConfig, BaseChartOptions, ChartTabularData } from '@carbon/charts'

	interface DispatchedEvents {
		load: null
		update: { data: ChartTabularData; options: T }
		destroy: null
	}

	type CoreChartClass = new (holder: HTMLDivElement, chartConfigs: ChartConfig<T>) => Charts

	const chartHolderCssClass = 'cds--chart-holder' // Used by Carbon Charts CSS
	export let data: ChartTabularData = [] // Chart data, default is empty
	export let options: T = {} as T // Specific chart option type, default is empty
	export let Chart: CoreChartClass // Used to instantiate next property
	export let chart: Charts | undefined // Instance of the chart class
	export let ref: HTMLDivElement | undefined // Binding to chart 'holder' in template below
	export let id = `chart-${Math.random().toString(36)}` // id for chart holder element

	const dispatch = createEventDispatcher<DispatchedEvents>()

	onMount(() => {
		try {
			chart = new Chart(ref as HTMLDivElement, { data, options })
			dispatch('load')
		} catch (error) {
			console.error('Failed to initialize chart:', error)
		}
	})

	afterUpdate(() => {
		if (chart) {
			try {
				chart.model.setData(data)
				chart.model.setOptions(options)
				dispatch('update', { data, options })
			} catch (error) {
				console.error('Failed to update chart:', error)
			}
		}
	})

	onDestroy(() => {
		if (chart) {
			dispatch('destroy')
			// Like core's Chart.destroy() but keeps div chart holder bound to ref as it's part of this template
			chart.components.forEach(component => component.destroy())
			chart.model.set({ destroyed: true }, { skipUpdate: true })
			chart = undefined
		}
	})
</script>

<div {id} bind:this={ref} class={chartHolderCssClass} {...$$restProps}></div>

2. Build with svelte-preprocess.
3. Look at output...

<script generics="T extends BaseChartOptions">import { onMount, afterUpdate, onDestroy, createEventDispatcher } from 'svelte';
const chartHolderCssClass = 'cds--chart-holder'; // Used by Carbon Charts CSS
export let data = []; // Chart data, default is empty
export let options = {}; // Specific chart option type, default is empty
export let Chart; // Used to instantiate next property
export let chart; // Instance of the chart class
export let ref; // Binding to chart 'holder' in template below
export let id = `chart-${Math.random().toString(36)}`; // id for chart holder element
const dispatch = createEventDispatcher();
onMount(() => {
    try {
        chart = new Chart(ref, { data, options });
        dispatch('load');
    }
    catch (error) {
        console.error('Failed to initialize chart:', error);
    }
});
afterUpdate(() => {
    if (chart) {
        try {
            chart.model.setData(data);
            chart.model.setOptions(options);
            dispatch('update', { data, options });
        }
        catch (error) {
            console.error('Failed to update chart:', error);
        }
    }
});
onDestroy(() => {
    if (chart) {
        dispatch('destroy');
        // Like core's Chart.destroy() but keeps div chart holder bound to ref as it's part of this template
        chart.components.forEach(component => component.destroy());
        chart.model.set({ destroyed: true }, { skipUpdate: true });
        chart = undefined;
    }
});
</script>

<div {id} bind:this={ref} class={chartHolderCssClass} {...$$restProps}></div>

Expected behavior

1. Expected generics="T extends BaseChartOptions" to be removed from line 1.
2. Expected there to be a line break before the first import statement.
3. Expected lang="ts" would be removed (my example uses lang="typescript" which is removed)
4. Expected formatting to use prettier

Information about your project:

• Your browser and the version: Google Chrome 133

• Your operating system: macOS 15.3.1

• svelte-preprocess version: 6.0.3

• Svelte 5.20.1 (compatibility mode), SvelteKit 2.17.2 (building with svelte-package)