Add watch directory option into setting files.

[mantaroh]

Is your feature request related to a problem? Please describe.

If we have a i18n.yml on top directory, spear-cli doesn't watch this file due to out of watch directory.
https://github.com/unimal-jp/spear/blob/5ff8cd2a36cc2168c4f3482cfb9d0553e2ed70e4/packages/spear-cli/src/magic.ts#L218-L221

Describe the solution you'd like

Spear allow specifying watch directory on spear.config.mjs.

Describe alternatives you've considered

Nope.

Additional context

Nope.

[mantaroh]

Document issue is https://github.com/unimal-jp/spear-doc/issues/1.