Rework virtual files

Before Volar, the ESM, JSX, and markdown parts of virtual files had to
map character by character, with parts shadowed by whitespace
characters. This was still how virtual files were built. However,
because of how Volar mappings work, this is unnecessary.

This change overhauls how virtual files are generated. This leads to
more flexibility of virtual files, meaning we can omit or inject
content.

This is already leveraged in several ways:

- Markdown nodes in MDX are represented by TypeScript fragments. This
  provides compatibility with JSX elements that require children on a
  type level.
- Markdown text nodes in MDX are represented by TypeScript JSX
  expressions containing an empty string.
- JavaScript chunks are represented in the virtual markdown files as
  HTML comments.