Add "no-indistinguishable-colors" to general config

[Volker-E]

From a UI Standardization perspective no-indistinguishable-colors is a powerful addition to combat the little annoyances when there are too many similar color definitions at too many different rules.
Let's add "no-indistinguishable-colors": [ true, { "threshold": 1 } ]

Threshold set to 3 is the default, 1 seemed to be the right one for our current color shades in my test runs (OOjs UI, Echo).

[jdforrester]

This has now been deprecated by upstream, who suggest someone goes and makes a plugin. :-(

[jitendravyas]

Also looking for a plugin for this