Adding a custom .pt grammar as an alternative to dataclass style declarations

[Intuity]

This PR is a significant extension to Packtype, adding an entirely custom grammar for declaring packages, constants, enums, structs, and unions.

• Adds a custom EBNF grammar using Lark;
• Adds a transformer to map Lark parser output onto Packtype's internal types;
• Adds unit tests to exercise custom grammar and error cases;
• Adds .pt variants to all compatible examples;
• Adds Packtype grammar definitions to each page of documentation;
• Bumps version to 3.0.0 (major change);
• Restructures CLI commands to allow multiple .pt files to be specified;
• Extends utils to wrap many of the _pt_ methods.

[Copilot]

Pull Request Overview

Adds support for a custom .pt grammar alongside dataclass-style declarations for Packtype, updates the VS Code extension metadata, enhances the CLI to accept multiple specification files and name‐filtering options, and adjusts tests and docs to reflect the new grammar.

• Introduced Lark grammar and VS Code syntax highlighting for .pt files
• Enhanced CLI (__main__.py) to load multiple spec files and apply naming filters
• Updated tests, examples, and documentation to cover and demonstrate the new .pt syntax

Comments suppressed due to low confidence (3)

docs/syntax/union.md:94

• The description mentions "struct" but this is in the union documentation. Change to "bit width of the union."

 * `<UNION>._pt_width` - property that returns the bit width of the struct;

vscode/packtype/language-configuration.json:3

• JSON does not support comments; these // lines will break parsing. Remove or convert them to valid JSON keys.

        // symbol used for single line comment. Remove this entry if your language does not support line comments

tests/integration/test_sv.py:23

• The test moved the file‐path argument after the code subcommand, changing the CLI invocation order and likely breaking the test. The .pt file path should appear immediately after the --debug flag.

            (resources / "test_pkg.py").as_posix(),