Fix "\\{{" immediately following "\{{"

Escaped-escape mustaches ("\\{{") immediately following escaped mustaches ("\{{") were being handled incorrectly.

For example, with context `{foo: 'what'}`, template `\{{foo}} \\{{foo}}` was being rendered as `{{foo}} \{{foo}}`, as opposed to the desired `{{foo}} \what`. 

This fixes the lookahead to make sure yytext still contains the appropriate slashes when we pop out of <emu> so they can be handled consistently by the initial state.