support variadic target arguments

[younggglcy]

Closes #340.

What this changes

Mage now discovers exported targets whose final parameter is ...string and
passes the remaining command-line tokens to it unchanged. A variadic target is
terminal for the invocation, so existing fixed-arity target chaining keeps its
current behavior and Mage does not need to guess boundaries from target names.

Pointer-style optional arguments may precede the variadic tail. In that form,
Mage parses flags until the first non-option token; a target-local -- can be
used when the tail itself starts with -. This separator is intentionally
scoped to optional-plus-variadic targets: pure variadic targets still receive
-- literally, and non-variadic target behavior is unchanged.

The parser metadata is shared by generated dispatch and help output, and the
website documentation describes the same terminal-target and pass-through
semantics. Unsupported non-string variadics remain hidden. Alias conflicts are
reported only when this feature makes a variadic target newly discoverable, so
historical fixed-arity alias behavior is preserved.

Example

func Run(name string, verbose *bool, args ...string) error {
	// ...
	return nil
}

mage run worker -verbose -- -race ./...

Validation

• go test -vet=all -tags CI -race ./...
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.2 run
• Go 1.18.10 coverage for the parser and variadic dispatch tests