Validate -main-is flag using parseIdentifier

This patch aims to validate the input given to the -main-is flag by parsing the input and validating that it is a (qualified) identifier or a module.

Fixes #24368 (closed)

requested review from @mpickering

added Blocked on Review label

added user-facing label

assigned to @mpickering

requested review from @AndreasK and removed review request for @mpickering

approved this merge request

assigned to @marge-bot and unassigned @mpickering

LGTM thanks for this!

removed Blocked on Review label

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Merged in 3d0806fc

closed