This PR fixes several issues in the Elixir lexer's sigil processing to match the Elixir language specification.

Changes

Fixed interpolation detection

• Lowercase sigils (~s, ~r, ~c, ~w) support interpolation
• Uppercase sigils (~S, ~R, ~C, ~W) do not support interpolation

In particular, uppercase letters sigils do not perform interpolation nor escaping. For example, although both ~s and ~S will return strings, the former allows escape codes and interpolation while the latter does not:

Ref: Interpolation and escaping in string sigils

Fixed sigil pattern regex

• Changed from ~([a-zA-Z])? to ~([a-z]|[A-Z][A-Z0-9]*)
• The sigil letter is now required (not optional), preventing errors when processing invalid patterns like ~()
• Properly matches custom sigils like ~CUSTOM or ~ABC123

Custom sigils may be either a single lowercase character, or an uppercase character followed by more uppercase characters and digits.

Ref: Custom sigils

Reference

• https://hexdocs.pm/elixir/sigils.html