General: Ensure wp.template retains its Underscore dependency

[Sukhendu2002]

Trac ticket: https://core.trac.wordpress.org/ticket/38052

What this PR does

Captures the Underscore instance when wp-util initializes. This prevents lazy wp.template() compilation from using a global _ later replaced by Lodash or another script.

Also adds a QUnit regression test that replaces global _ after initialization and confirms template rendering still works.

Use of AI Tools

AI assistance: Yes
Tool(s): pi
Model(s): GPT-5.6
Used for: Code review; final implementation and tests were reviewed and edited by me.

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