Replace parser gem with prism for Witchcraft AST parsing

[t6n]

Description

Motivation

The parser gem only supports Ruby syntax up to version 3.3 and will not receive updates for Ruby 3.4+. Starting with Ruby 3.4, Prism is the
official parser built into Ruby itself and is the recommended tool for all AST-related operations going forward.

This change future-proofs the Witchcraft feature by switching to Prism before parser becomes incompatible with newer Ruby versions.

Changes

• Replace require 'parser/current' with require 'prism'
• Use Prism::Translation::ParserCurrent.parse instead of Parser::CurrentRuby.parse
• Update check_requirements! to check for the Prism constant instead of Parser

Technical Notes

Prism provides a translation layer (Prism::Translation::ParserCurrent) that produces Parser::AST::Node objects identical to those from the parser
gem. This means:

• All existing AST transformation methods continue to work unchanged
• unparser remains fully compatible since it receives the same node types
• No changes required to the AST manipulation logic (replace_lvar, replace_send, replace_self, etc.)

Dependencies

The Witchcraft feature now requires prism instead of parser as a direct dependency. Note that unparser still has parser as a transitive
dependency for its AST node classes, but this is handled internally and does not affect Chewy's public API.

[bbatsov]

Yeah, that's a reasonable idea. I'll review it more carefully soon.

[bbatsov]

Master has been updated with CI fixes and compatibility changes (#998) — we now target Ruby 3.2+ and Rails 7.2+. Could you rebase this PR on top of master so CI can run properly? Thanks!

[bbatsov]

This is a great change — parser won't support Ruby 3.4+ and Prism is the right path forward.

[bbatsov]

One concern: Prism is bundled with Ruby 3.3+ but not available in Ruby 3.2, which we still support (required_ruby_version = '~> 3.2'). This PR would break Witchcraft on 3.2 unless we either:

1. Add prism as a gem dependency (not ideal for an optional feature)
2. Try to require 'prism' and fall back to parser if unavailable
3. Wait until we drop Ruby 3.2 support

What do you think? Option 2 seems the most pragmatic if you want to keep the PR alive now.

[t6n]

@bbatsov thanks for reviewing. I added a fallback to parser.

[t6n]

@bbatsov It would be great if this PR could be included in version 8.0

[bbatsov]

It's a bit late for Chewy 8.0, but this will land in 8.1. (hopefully soon)