Fix cast crash that could happen in markdown blockquote

After 0.19.1, this started crashing:

```
> *Note*
> ...
```

due to a force cast I was doing foolishly (which broke because "*Note*" is an
EmphasisNode, not a Text node)

Changed logic from `child as Text` to `child as? Text`

tools/gradle-plugins/extensions/markdown/src/main/kotlin/com/varabyte/kobwebx/gradle/markdown/handlers/Blockquote.kt

@@ -58,8 +58,9 @@ fun SilkCalloutBlockquoteHandler(
 ): NodeScope.(BlockQuote) -> String {
     return { blockQuote ->
         val silkCallout = blockQuote.firstChild.firstChild?.let { firstChild ->
-            firstChild as Text
-            val regex = """\[!([^ ]+)( "(.*)")?]""".toRegex()
+            @Suppress("NAME_SHADOWING")
+            val firstChild = firstChild as? Text ?: return@let null
+            val regex = """\[!([^ ]+)( "(.*)")?]""".toRegex() // [!TYPE "LABEL"]
             val typeMatch = regex.find(firstChild.literal) ?: return@let null
             firstChild.literal = firstChild.literal.substringAfter(typeMatch.value)