feat: expose public API for custom syntax parsers

[cxxxr]

Summary

Currently, implementing a custom syntax parser (like tree-sitter integration) requires using internal symbols:

• lem/buffer/internal::syntax-parser - base class for custom parsers
• lem/buffer/internal::%syntax-scan-region - method to implement syntax scanning

Background

In PR #2046 (tree-sitter integration), we needed to create a custom syntax parser that integrates with Lem's syntax highlighting system. While we refactored to use public APIs for text property manipulation (put-text-property, remove-text-property, text-property-at), the parser class itself still requires internal symbol access.

Current Implementation

;; extensions/tree-sitter/integration.lisp
(defclass treesitter-parser (lem/buffer/internal::syntax-parser)
  ...)

(defmethod lem/buffer/internal::%syntax-scan-region ((parser treesitter-parser) start end)
  ...)

Proposal

Export a public API for custom syntax parsers:

1. Export syntax-parser class from lem or lem-core package
2. Export %syntax-scan-region generic function (possibly renamed to syntax-scan-region)
3. Document the protocol for implementing custom syntax parsers

Benefits

• Enables extensions to implement custom syntax highlighting without internal symbol access
• Follows Lem's convention of using exported symbols
• Makes the syntax parser extension point official and stable

Related

• PR feat: add tree-sitter based syntax highlighting support #2046: tree-sitter syntax highlighting integration