Feature: Highlight constants

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Let syntax highlighter colorize constants using the same rules as for values they represent.

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This is an example of semantic highlighting: requires scanning prior source code and collecting constant symbols. Can be CPU consuming. Other elements get coloring purely after their syntax.

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The editor will rescan the document on open and save events. If you added or removed a new constant, it won't be reflecting in highlighting until you save the document. It saves a lot of resources on background scanning.