[playground][BUG] frontend and backend parse mustaches differently

[RogerHYang]

To reproduce, enter three pairs of braces, e.g.

{{{x}}}

The frontend thinks the variable name is {x} but the backend thinks it's x, resulting in a mismatch.

[cephalization]

I was afraid of this when duplicating parsing logic between FE and BE. Will have to more closely observe the spec that BE is applying for each language and update the lezer grammars