[feature] global config / way to add languages globally

[pbsds]

⭐ Suggestion

I want to be able to register additional tree-sitter languages, to be used in ad-hoc invocations of sg. This can be realized in multiple ways, number 2 or 5 of which i think is best.

1. Add support for global a $HOME/.config/ast-grep/sgconfig.yaml
2. Add support for reduced scope $HOME/.config/ast-grep/extra-languages.yaml
3. Add an environ, e.g. AST_GREP_LANGUAGES="lang:/path/to/grammar-lang.so;"
4. Make all sg subcommands, not just sg scan, accept a --config, allowing me to shadow sg with a shell alias
5. Allow one to link against additional languages at compile time

related: #30 (comment)

💻 Use Cases

• Ad-hoc/non-project uses, or where checking a sgconfig.yaml into the repo is undesirable