Fix Elixir v1.19 warnings

[ianko]

Description

This PR fixes warnings that appear when running on Elixir 1.17+ and 1.19+, addressing both deprecated syntax and new type system checks.

Issues Fixed

1. Elixir 1.17+ Charlist Deprecation Warnings

In Elixir v1.17, single-quoted charlists were hard deprecated. The warning states:

"using single-quoted strings to represent charlists is deprecated. Use ~c"" if you indeed want a charlist or use "" instead."

Changes:

• Updated test/xpeg_test.exs and test/examples_test.exs to use ~c sigil
• Replaced patterns like 'a' → ~c"a" and {'a'..'z'} → {~c"a"..~c"z"}

2. Elixir 1.19 Type System Warnings (Arity Mismatch)

Elixir v1.19 introduced enhanced type inference for anonymous functions, which detected an arity mismatch in the code generator:

     warning: expected a 2-arity function on call:

         func.(captures, ctx)

     but got function with arity 1:

         (non_empty_list(term(), term()) -> dynamic(non_empty_list(term(), term())))

     where "func" (context Xpeg.Codegen) was given the type:

         # type: (non_empty_list(term(), term()) -> dynamic(non_empty_list(term(), term())))
         # from: deps/xpeg/lib/codegen.ex:181
         func = fn [date, op | cs] -> [[date, op] | cs] end

     typing violation found at:
     │
 185 │                 true -> func.(captures, ctx)
     │                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     │
     └─ deps/xpeg/lib/codegen.ex:185: :"{:__aliases__, [counter: {Policy.Engine.PolicyParser, 2}, line: 21, column: 16], [:Policy]}-#Reference<0.4023179176.3650617345.76900>".parse/8

Root Cause:
The code was conditionally calling functions with different arities at runtime based on the userdata option, but generating the same call site.

Solution:
Split code generation into separate branches at compile-time (commit bab109d). Now the compiler generates the correct function call for each case:

• With userdata: true → generates func.(captures, ctx) (2-arity)
• Without userdata → generates func.(captures) (1-arity)

This eliminates the runtime conditional and allows the type system to correctly verify each path.

Testing

• ✅ All 24 tests pass
• ✅ Zero compilation warnings on Elixir 1.19
• ✅ Type system correctly verifies function arities
• ✅ Functionality preserved - all existing behavior maintained