Format selection (VHDL, possibly other languages as well)

[ckuhlmann]

Is your feature request related to a problem? Please describe.
When a file contains parts that are not handled well by automatic formatting (e.g. formatting of large and nested if conditions that are arranged over several lines for clarity and logically indented), it would be nice to still be able to format some sections of the document

Describe the solution you'd like
If the user selects part of the document, there should be a "Format selection" command or a mode of the "Format document" command, that affects only the selected text.

Describe alternatives you've considered
Currently, I use the formatter of p2l2.vhdl-by-hgb (https://github.com/g2384/VHDLFormatter) in a separate VSCode portable install since its formatter works with partial VHDL files, but this requires lots of copy-pasting and two VSCode instances, which gets confusing.