feat: update ParseCursor

[ianlewis]

Description:

• Rename ParserContext to ParseCursor and separate it from the context.Context.
• Add a NewParseCursor to allow creating a ParseCursor. This should be useful for testing parse functions individually.
• Rename CustomLexerCursor to LexCursor to make it a little less long-winded. Also rename NewCustomLexerCursor to NewLexCursor.

Related Issues:

• [feature] improve testability #188

Checklist:

• Review the CONTRIBUTING.md documentation.
• Add a reference to a related issue in the repository.
• Add a description of the changes proposed in the pull request.
• Add unit tests if applicable.
• Update documentation if applicable.
• Add a note in the CHANGELOG.md if applicable.

[Copilot]

Pull request overview

This PR updates the lexing/parsing public APIs to improve testability by renaming the cursor/context types and passing context.Context separately.

Changes:

• Renamed ParserContext → ParseCursor and updated ParseState.Run to accept (context.Context, *ParseCursor[V]).
• Renamed CustomLexerCursor → LexCursor and updated LexState.Run signatures accordingly.
• Updated examples/tests/docs/changelog to use the new cursor types and constructors (NewParseCursor, NewLexCursor).

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
parser.go	Introduces ParseCursor, updates ParseState API, and wires Parser.Parse to pass ctx + cursor.
custom.go	Renames lexer cursor to LexCursor, updates LexState function types and lexer execution.
README.md	Updates documentation/examples to reflect the new cursor types and parsing API.
CHANGELOG.md	Notes the renames in the Unreleased section.
template_example_test.go	Updates template lexer/parser example code to the new cursor APIs.
ini_example_test.go	Updates INI lexer/parser example code to the new cursor APIs.
infix_example_test.go	Updates Pratt parser example to accept context.Context + ParseCursor.
lexparse_test.go	Updates parser state implementations and lexer state signatures to the new APIs.
parser_test.go	Refactors parser tests to construct/use ParseCursor explicitly.
custom_test.go	Refactors lexer cursor tests to use LexCursor/NewLexCursor.

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