Don't indent multiline strings when using tabs as indentation

[tb148]

Nix currently doesn't remove leading tabs in multiline strings. As such, indenting those strings with tabs changes semantics. To fix this, we can either specify the number of spaces for these strings specifically (with doesn't always work, as one of the many reasons for using tabs as indentation is that the reader can decide how long indentation should be) or just not indent them at all.

[tb148]

I believe the modifications needed are as follows:

• On line 71, use trim_start_matches to only count spaces as indentation.
• On line 103, return "" instead of "\t".
• On line 123 and line 132, add a condition that says the indentation config is not set to Tabs.

However, I have never written rust before, so I'm not sure if I'm missing something.

[tb148]

I've noticed that the string tests contains one example where semantics is changed.