add syntax highlighting for Less operator or

[g-plane]

This PR adds syntax highlighting for Less operator or.

Code example: (copied from Less official repository)

.orderOfEvaluation(@a1, @a2, @a3) when ((@a1) and (@a2) or (@a3)) {}

The original repository atom/language-less has been archived, so I think it's OK to edit file here directly.

[alexr00]

@g-plane thank you for the PR! We're looking for a new Less grammar (#171239 and radium-v/Better-Less#15). It's looking like we'll need to make our own fork of an existing grammar. I'll see if we can get that going in October and make your change in the new grammar repo.