Add Rust API for parsing with scope locals (eval/binding support)

[sisshiki1969]

Hi everyone — monochrome here, building a Ruby implementation + JIT in Rust from scratch over at monoruby.

Motivation

I recently switched monoruby's parser to Prism, but I had to use Prism from a fork because there's no Rust-side API for passing scope locals (the surrounding binding's variable names) into the parser. That's needed for Kernel#eval / Binding#eval, where an identifier might be either a local from the outer scope or a method call — Prism needs the local names to disambiguate.

The C side already has all the machinery (pm_options_*, pm_options_scope_t); it just isn't exposed through ruby-prism-sys / ruby-prism.

Proposed change

ruby-prism-sys:

extend the bindgen allowlist for the missing symbols (pm_options_free, pm_options_line_set, pm_options_scope_init, pm_options_scope_forwarding_set, pm_options_scopes_init, pm_string_constant_init).

ruby-prism:

add Options, Scope, and parse_with_options().

Usage on the Rust side:

let options = prism::Options::new().line(line).scopes(scopes);
let result = prism::parse_with_options(code.as_bytes(), options)

A working version is on the monoruby branch of sisshiki1969/prism (~240 lines vs v1.9.0, no C / build-script changes).
Diff is here.

If this looks reasonable I'd be happy to send a PR.
Any feedback on the API shape before I do?