support frontmatter syntax

[m4rch3n1ng]

fixes #253
fixes #224

this parses rust frontmatter as specified in rfc 3503. like with raw strings (which this implementation takes quite a bit of inspiration from), the frontmatter content is parsed as a seperate node (frontmatter_content) to allow, for example, to add a toml language injection for the content.

i had to do a little hack because of tree-sitter automatically stripping all whitespace, meaning that i cannot always rely on having a newline before the ending fence. this means that it will incorrectly parse some things, but i think those are edge cases that are acceptable, because i couldn't figure out a way to make it stop stripping all whitespace between _frontmatter_start and frontmatter_content (which is also the cause for #251):

// this is technically valid rust and rustc doesn't consider the indented line
// as the ending fence, but instead the third line, while this pr already bails on the second line.
---
  ---
---

fn main() {}

no diff for rustc tests as rustc support for it is not yet merged.