Merge #10211

10211: assists: Promote module to folder r=jonas-schievink a=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.

![未命名(1)](https://user-images.githubusercontent.com/13777628/132958377-14555d6f-a64a-4b9b-9154-90a3b86fd685.gif)

Co-authored-by: longfangsong <longfangsong@icloud.com>

diff --cc crates/ide_assists/src/assist_context.rs
Simple merge

diff --cc crates/ide_assists/src/lib.rs
Simple merge

diff --cc crates/ide_assists/src/tests/generated.rs
Simple merge