Parser crashes on charlist with pure interpolation ('#{x}')

[moxley]

Spitfire 0.3.11 raises FunctionClauseError when parsing a charlist literal whose only content is a string interpolation segment.

Repro

Mix.install([{:spitfire, "~> 0.3"}])
Spitfire.parse(~S|defmodule D, do: '#{x}'|)

Stack trace

** (FunctionClauseError) no function clause matching in String.to_charlist/1
    # 1
    {{1, 19, nil}, {1, 22, nil}, [{:identifier, {1, 21, ~c"x"}, :x}]}
    (elixir 1.17.3) lib/string.ex:2847: String.to_charlist/1
    (spitfire 0.3.11) lib/spitfire.ex:2480: Spitfire.parse_string/1

Root cause

The :list_string non-interpolation clause of parse_string/1 (lib/spitfire.ex:2480) is missing the when is_binary(string) guard that the parallel :bin_string clause at line 2463 has. When the tokenizer emits a single-element token list for a charlist that is pure interpolation, the unguarded clause matches and String.to_charlist/1 is called on the interpolation-segment tuple.

Charlists with surrounding text ('foo#{x}bar') were unaffected because their token list has multiple elements and falls through to the interpolation-aware clause.

Scope

Input	0.3.11
'foo'	OK
'foo#{x}bar'	OK
'#{x}'	crash
"#{x}"	OK
heredoc charlist	OK
heredoc bin	OK

Also affects downstream xp_spitfire (vendored by Expert LSP) and breaks
Go-to-Definition on files containing this construct. Hit naturally in e.g.
plug_forwarded_peer's lib/plug_forwarded_peer.ex.