parser: fix Base.parse and add errors and diagnostics

[apiology]

Claude: This PR was written by Claude Code on behalf of @apiology.

Problem:

Steep rejects documented parser calls:

# test.rb:79:29: [error] Type `::Parser::Base` does not have method `new`
parser = Parser::CurrentRuby.new

begin
  Parser::CurrentRuby.parse("foo(")
# test.rb:94:15: [error] Cannot find the declaration of constant: `SyntaxError`
rescue Parser::SyntaxError => error
  error.diagnostic
end

Code that builds its own parser, configures diagnostics or rescues syntax errors cannot be type-checked.

Solution:

Declare CurrentRuby as singleton(Parser::Base), make Base.parse/parse_with_comments class methods taking a String and the instance versions take a Source::Buffer, and add SyntaxError, UnknownEncodingInMagicComment, Diagnostic, Diagnostic::Engine, Base#diagnostics and Base#version, each matching parser v3.3.12.0.

Base#version is defined in each RubyNN parser, not in base.rb; Base#initialize calls it, so it is declared on Base.

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parser

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[kozy4324]

APPROVE

[github-actions]

Thanks for your review, @kozy4324!

@apiology, @kozy4324 This PR is ready to be merged.
Just comment /merge to merge this PR.

[apiology]

/merge