Add support for optional arguments

[natefinch]

This update adds support for optional arguments, which are indicated in the go code by pointer arguments. In the CLI, you must use -arg=value semantics to specify optional values (or just -arg if the argument is a boolean and you want to specify true).

[Copilot]

Pull request overview

Adds “optional argument” support to mage targets by treating pointer-typed parameters as optional and parsing them from -name=value flags, with corresponding updates to help/usage output and new test coverage.

Changes:

• Extend parse metadata to mark pointer parameters as optional and generate execution code that parses -name=value options into pointer variables.
• Update generated help/usage to show required args positionally and optional args in bracketed [-name=<type>] form.
• Add parse- and invocation-level tests plus test magefiles/testdata demonstrating optional arguments across supported types.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
site/content/targets/_index.en.md	Documents optional arguments via pointer params and -name=value CLI syntax.
parse/testdata/optargs.go	New parse testdata covering optional args across supported base types.
parse/parse_test.go	Adds unit test asserting optional args are detected and typed correctly.
parse/parse.go	Adds Arg.Optional and generates optional-arg parsing code + usage helpers (Required/Optional args).
magefile.go	Removes previously defined alias/target sample code from repo magefile.
mage/testdata/optargs/magefile.go	New runtime test magefile demonstrating optional args and chaining behavior.
mage/template.go	Updates help output and required-arg counting to account for optional args.
mage/args_test.go	Adds end-to-end tests for optional args parsing, errors, chaining, and help output.

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