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.

[wetneb]

This PR is also integrated in the tree-sitter-rust-orchard fork. See #271 (comment) for context.

[nik-rev]

I suggest making the "Info String" a separate node, similar to how it is in markdown. so you can match against it like

(info_string "yaml")

To inject the "yaml" language for example.

[m4rch3n1ng]

that is actually surprisingly hard, as that is whitespace-sensitive (an "--- info string" should parse differently than a "---\ninfo string" and the tree-sitter-rust grammer automatically strips all whitespace between nodes, see:

tree-sitter-rust/grammar.js

Line 64 in 3691201

/\s/,

i have a kinda-working prototype, but that one doesn't work for incremental parsing. i'll try again later to get it actually working, but don't get your hopes up lol.