Add a lint for named colors as map keys

[nex3]

A mistake we see occasionally is users using unquoted strings as map keys, having one of those keys be named something like blue, and getting confused later on when that key compiles to #00f instead. We should add a lint rule to help out with this.

The most conservative way to do this is probably to warn the user when a map:

• contains an unquoted string key,
• contains a color name key, and
• contains only unquoted string or color name keys.

Less conservatively, we could also warn about maps that have keys of mixed type in general or maps with any unquoted string keys, although this risks having more false positives.

[mik01aj]

I see you want to prevent https://github.com/sass/libsass/issues/652, but wouldn't https://github.com/sass/linter/issues/12 also solve it in a more generic way?

Also, do these examples describe what you expect here?

$theme1: (primary: #f00, secondary: #00f); // ok
map-get($theme1, primary); // ok
map-get($theme1, 'primary'); // ok

$theme2: (red: #f00, blue: #00f); // linter error
map-get($theme2, red); // linter error
map-get($theme2, 'red'); // ok (although this won't work, afaik)

$theme3: ('red': #f00, 'blue': #00f); // ok
map-get($theme3, red); // linter error
map-get($theme3, 'red'); // ok

[nex3]

I see you want to prevent sass/libsass#652, but wouldn't #12 also solve it in a more generic way?

Yes, it is kind of redundant with #12. It may still be useful to have both, though, for users who find #12 to be too onerous and disable it.

Also, do these examples describe what you expect here?

$theme1: (primary: #f00, secondary: #00f); // ok
map-get($theme1, primary); // ok
map-get($theme1, 'primary'); // ok

$theme2: (red: #f00, blue: #00f); // linter error
map-get($theme2, red); // linter error
map-get($theme2, 'red'); // ok (although this won't work, afaik)

$theme3: ('red': #f00, 'blue': #00f); // ok
map-get($theme3, red); // linter error
map-get($theme3, 'red'); // ok

For this and other map key lints, I was just thinking that it would affect literal maps, not map-get() calls.