feat: add opt-in metavars to flag help

[integrii]

Adds opt-in metavars for non-toggle flags in help output.

Changes:

• Derives metavars from long names, with short-name fallback.
• Keeps metavars disabled by default.
• Adds parser and package-level enable/disable helpers.
• Leaves boolean toggles without metavars.
• Documents the behavior and adds focused tests.

Related to #105.

Validation: goimports, gofmt, go vet ./..., go test ./...

[integrii]

Added a regression test for the rendered columns. With metavars enabled, every long flag and every description now shares one global start column, including built-in --version/--help and flags with or without short names.

[flaggy-community-bot]

Review complete: no blocking correctness, regression, security, or maintainability findings. The opt-in default preserves existing help output; bool and []bool toggles remain without metavars; long-name and short-only derivation plus alignment are covered by tests. Validation passed: go test ./..., go vet ./..., gofmt -l ., and git diff --check. Recommendation: merge.