feat: Add chezmoi:template:format-indent template directive

[twpayne]

Fixes #3873.
Implements @bradenhilton's suggestion in #3875 (comment).

This still needs some work, so this is draft PR.

[halostatue]

The changes look good. I haven't tested them myself, but I think it's worth asking whether we want to (1) have two directives instead of one, and (2) be strict on string values to prevent footguns.

[halostatue]

I wonder if this might not be better as two directives, format-indent-width and format-indent. They would be mutually exclusive. format-indent-width would be the integer number of spaces to use per level; format-indent would a a string, but would have to match ^[ \t]+$ to be valid. That is, format-indent="" would not be valid, nor would format-indent="..".

(One could argue that it should be (^ +$|^\t+$) to prevent mixing tabs and spaces.)

[twpayne]

Thanks, done. I've not been super-strict with the parsing, but any errors should be quickly visible to the user.

[bradenhilton]

If we want to be strict about only allowing tabs or spaces, would something similar to how it's done in EditorConfig be better? i.e.

chezmoi:template:indent-style:spaces
chezmoi:template:indent-style:tabs

chezmoi:template:indent-size:4

# and possibly also
chezmoi:template:tab-width:4

[twpayne]

I don't think we need to be super-strict on this, and that it should be sufficient to need only one directive to specify the indent. So, I'll leave this as-is.

[halostatue]

Looks good.