Add support for defining .ts entry points through block.json assets

[fabiankaegy]

For working with blocks we handle all the asset registration though the block.json file and it's script,viewScript, viewScriptModule, editorScript, and scriptModule keys.

These keys support relative file: imports which then get registered as webpack entrypoints.

The problem is that the block.json file just gets copied into the dist directory without getting transformed. So if the import is a .ts file that gets transpiled by webpack into a .js file the import would fail.

We should add a string transform for .ts/tsx to .js