REST API: Match REST API routes on namespace before performing regex checks.

Rule out groups of API endpoints by simple namespace string comparison to reduce the number of regex checks necessary when matching a route.

Props TimothyBlynJacobs.
Fixes #48530.

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git-svn-id: https://core.svn.wordpress.org/trunk@47060 1a063a9b-81f0-0310-95a4-ce76da25c4cd