Render whitespace

[Omnikar]

Closes #1068 (intended config behaviour described there).

Currently only "none" and "all" are supported; I still need to figure out what to do to be able to render only whitespace in selections.

To-do:

• Implement "selection" to render whitespace that falls into a selection.
• Add colors for rendered whitespace to themes.
• Allow the characters used to represent whitespace to be configured.

[valpackett]

The characters should be configurable too, and tabs should be included. e.g. in kakoune I have

add-highlighter global/unicode-whitespace show-whitespaces -tab '▏' -nbsp '␣' -spc ' '

[Omnikar]

@unrelentingtech OK, I added that as a todo

[archseer]

Yeah I was thinking this could be a general substitution map:

{ " " = "."} etc. Looks like I forgot to comment on the original issue. But on a second thought that would result in a map lookup per character which isn't great.

Might be able to optimize by using a Vec<Option<char>> where the index would be the ASCII code. That would limit substitutions to ascii chars but I think that's enough.

[archseer]

Alternatively we could have a hard-coded set of characters we support mapping.

[archseer]

See also how vim does this http://vimdoc.sourceforge.net/htmldoc/options.html#'listchars'

[archseer]

Replaced by #1802