fix(ruby): scope string overrides to literal content

[vitallium]

The Ruby extension currently captures each entire string with:

(string) @string

A Ruby string can contain executable Ruby code:

"The result is #{user.name}"

The broad capture also applies the string scope to user.name inside the interpolation. This can enable string-specific settings in normal Ruby code.
Changing the query to:

(string
  (string_content) @string.inclusive)

fixes the problem.

This limits the string scope to literal content:

string
->string_content       -> string scope
->interpolation          -> normal Ruby scope

This keeps string-specific settings, including Tailwind language-server opt-in and completion query characters, inside Ruby string content. Ruby code inside #{...} keeps normal Ruby behavior.

Closes #313