Adding helper for not declared variables/functions

[angelator312]

When you have an undeclared variable, the editor can declare it for you.
That's really common thing.

[angelator312]

Also for undeclared function will be helpful.

[SomeoneToIgnore]

That's not what Zed editor can do, as it barely knows what a language is: it might capture certain variables in ast (based on tree-sitter queries that might be incomplete) but has no clue how to do a semantic analysis (to detect undeclared things) and no clue what the types are and how to use these.

For example, Rust language "knowledge" for Zed is https://github.com/zed-industries/zed/tree/main/crates/languages/src/rust

The rest of the language knowledge, including the requested features, come out of language servers: e.g. rust-analyzer is able to do something like that already via code actions.