Complete annotated grammar reference/spec from `grammar.ohm`

[novusnota]

Current grammar page doesn't bring much value, especially for those willing to work on Tact compiler development or related tooling.

The page should be moved to the Language → Reference section, and it should cover different parts of the official Tact grammar in Ohm with examples showcasing their usage in the language.

Now, it's possible to do it with syntax highlighting of Ohm code blocks thanks to the merged TextMate grammar for Ohm, brought from novusnota/vscode-ohm.

Reference links:

• https://craftinginterpreters.com/appendix-i.html
• https://c3-lang.org/references/docs/specification/