Improve merged file generation

[tobil4sk]

This removes a couple differences between the merged files generated here and the upstreamed files in the vim repository:

• Generated files are now correctly indented
• The merge.js script now also generates a typescriptcommon.vim file as well, which is what vim wants: https://github.com/vim/vim/blob/d33afe12c6639d70fca82230df6b9fdee7365423/runtime/syntax/shared/typescriptcommon.vim

[tobil4sk]

@HerringtonDarkholme Thanks. I wonder, is there any need to keep the merged/typescript.vim and merged/typescriptreact.vim files? If the purpose of these files is just to make it easier to upstream to vim then maybe only merged/typescriptcommon.vim is needed?

EDIT: I did a search and it looks like this was only added to support upstreaming to vim: #113.

[HerringtonDarkholme]

I seldomly get patches from upstream vim and vim has been diverging from this repo for a long time.
The necessity of this merging script is not obvious TBH.

[tobil4sk]

I seldomly get patches from upstream vim and vim has been diverging from this repo for a long time.
The necessity of this merging script is not obvious TBH.

As I've been working on my PRs I've been comparing against vim and vim hasn't diverged from this repo too much apart from a few changes that have been mentioned in the past (all solved in #288). The vim repo still says to come to this repo to contribute to the typescript syntax rather than making changes directly to the vim repo. The reason I found it useful to update this merge script is to make it easier to update the files in the vim repo with the recent changes here, so for me it seems like the necessity is still there.