Introduce `SyntaxHighlightingTokens` instead of `[SyntaxHighlightingToken]`

[ahoppen]

Born from #857 (comment).

If we introduce a SyntaxHighlightingTokens type, we could put the lspEncoded and mergingTokens methods on there instead of as an extension on Array< SyntaxHighlightingToken>, which would be nicer encapsulation.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://116705656