fix: env_var with (interpolated) string as value

[blindFS]

Fixes the parsing error described in blindFS/topiary-nushell#20

Known difference to native nu-parser after the fix:FOO=(bar) baz

1. error by tree-sitter-nu.
2. assign a string of '(bar)' (with the parentheses) to env variable FOO in nu

I guess that's a corner case nobody actually cares.

[fdncred]

kind of a weird scenario to have '(bar)'. of course, nushell would want to evaluate that. in nushell you'd probably have to do char lp and char rp.