lexer: skip string literals except in interpolation

[ikostia]

Summary

See commit da0ab7d with a test demonstrating the bug this PR fixes, but TL/DR is that single document on the position of a / in a string literal "/usr/bin/home" would return the division opeartor docs. This is not desired, of course.

It is important to note that deciding that's a correct fix here is tricky:

1. the lexer runs on a pre-ppx source and ocaml itself does not have any string interpolation, so %{name} in "hello %{name}" is just a bit of a string literal, as per the compiler
2. however, the PPXes do exist and are common, so from a practical perspective it makes sense to try and treat %{name} as an interpolation for the purposes of documentation/hover etc

I chose to prioritize 2, so this PR handles interpolation inside string literals and skips only non-interpolated bits of string literals.

Testing

Existing tests pass + failure demonstration test now tests the correct behavior