Introduce --only/--ignore in the shopify theme serve help message

[karreiro]

It's related to Shopify/cli#648

WHY are these changes introduced?

The shopify theme serve help message was not including the --only/--ignore flags.

WHAT is this pull request doing?

This PR introduces the --only/--ignore flags in the shopify theme serve help message.

How to test your changes?

• Run shopify theme serve -h

Post-release steps

None.

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above (if needed).