assists: Promote module to folder

[longfangsong]

Close part of #10143.

This PR adds a assist to promote module to directory, which means make a .rs file module into a directory style module with the same name.

[flodiebold]

I'd call this something like "Turn a.rs into a/mod.rs" to make clearer what it does. It's IMO not a "promotion" in any sense, and a mostly unnecessary transformation nowadays.

[longfangsong]

I'd call this something like "Turn a.rs into a/mod.rs" to make clearer what it does. It's IMO not a "promotion" in any sense

Intellij Rust and emacs uses the word "promote". Though I'm not sure whether it is a good name either, maybe we should keepace with them?

[flodiebold]

I assume both IntelliJ and Emacs had this function before the 2018 edition. At that time, you could view it as a 'promotion' since it wasn't possible to have submodules without it. Plus, I don't think reusing the terminology from them really helps anyone -- it's not like someone used to "promote" wouldn't understand "Turn a.rs into a/mod.rs".

[matklad]

I do think we need something like this, but I don't think it's a good idea to make it available anywhere in a module -- this adds a 💡 entry everywhere, which is unhelpful every time you don't invoke this assist.

Let's re-use @vsrs's brilliant insight in #9735 and make the assist avialable only when the whole file is selected (ctrl+a).

I've documented this and other tricks in matklad@46cdde7

[longfangsong]

Now this assist is avialable only when the whole file is selected.
/cc @bjorn3 @matklad

[jonas-schievink]

Thanks, looks great now!

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)

[lnicola]

changelog feature (first contribution) add assist to convert file module to directory

[lnicola]