feat(watchTriggerable): extending watch with a manual trigger

[LittleSound]

Description

When you want to manually call a watch / watchEffect that uses the onCleanup parameter; simply taking the WatchCallback out and calling it doesn't make it easy to implement the onCleanup parameter.

The watchTriggerable function is a secondary wrapper around watchIgnorable, which returns a trigger function that immediately executes a WatchCallback and solves the problem of implementing onCleanup.

Additional context

issue: #1712

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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