Update parser grammer bnf provided

[Tbusk]

Description

Hi, the provided bnf grammar file in the parser docs is quite dated, has typos, and incomplete sections. This should be updated.

Additionally, the commit reference at the top of the page no longer exists. The link to the commit should be found and added again, or just removed.

https://docs.vala.dev/contributor-guide/compiler-guide/03-00-the-vala-compiler/03-02-parser.html

Luckily, I have one I am working on that has a lot of support already. Once I work out the kinks and have full support, I'll open a PR.

The latest version I have can be viewed at https://github.com/Tbusk/vala-jetbrains-plugin/blob/main/src/main/java/com/tbusk/vala_plugin/syntax/Vala.bnf