Reorganizing for additional rust-analyzer extensions

[trevarj]

I should start by saying I'm very new to neovim and lua, but I'm trying to adopt it as my main editor for Rust dev. The extra rust-analyzer extensions are crucial for this, in my opinion. There are some very useful commands, like "expand macro" and "open cargo.toml"

In this PR, I implemented the open cargo file command and also reorganized the structure of the project to prepare for more rust-analzyer specific extensions. By moving the inlay-hints ext to the rust-analyzer module, it will break current configurations, but I think it's better for organization.

If this does get merged, I will gladly update the documentation with how to use the "open cargo file" function:

:lua require('lsp_extensions.rust_analyzer').open_cargo_toml()

...and any others that will be added

Please nitpick or correct on anything that I did wrong or out of convention 😊