Syntax: Add support for string interpolation

[amiralies]

I'm not sure about the grammar maybe take a look at syntax repo.
do we have $name or just ${name}?

[ryyppy]

$name should theoretically highlight as well, but I think we'd rather nudge ppl to always use the ${name} variant, since the former requires you to use the j` prefix.

[amiralies]

Added back single var syntax in pr