Pylance should support completions inside of strings

[rchiodo]

Pylance supports goto def inside of a string, which can be useful say when looking at installed apps in a Django app:

The next step I think would be to support completions while typing. These completions might have different activation keys, but you could:

• Complete module/type/attr names when '.' is hit
• Complete paths for strings assuming same folder when '/' is hit

[rchiodo]

Somewhat related:
#1351