Emphasize `coffeeInterpDelim` for more distinction from `String`

[summivox]

link coffeeInterpDelim to Operator instead of PreProc. In this way it's more likely to be colored differently from String in a variety of color schemes.

Edit: Rationale: "abc #{ expr } def" is just 'abc' + expr + 'def', so#{ and } are actually "+ and +", which look like operators to me. Normally String and Operator are contrasted, since string concatenation is frequent.