Separate out vim highlighting classes a bit

Change-Id: I33d22b04515f7c40eb382d5420919fa0add9a1de
Highlight strings and interpolations more clearly.

The " delimiters of strings are now highlighted, as are the \( and ) of string
interpolations. The contents of interpolations are not highlighted as string
constants, but as top-level syntax.

Change-Id: I7d034ca917d501a1f0af1da24d95e79cd3c65593
Add %: to syntax highlighting

Change-Id: I8165808c6824297be3316753aac39f931e072fa0