fix(internal): parse let directive alias as pattern instead of expression

[gtm-nayan]

HEADS UP: BIG RESTRUCTURING UNDERWAY

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Name	Status	Preview	Updated (UTC)
svelte-dev-2	❌ Failed (Inspect)		May 28, 2023 11:52am

[benmccann]

should this be optional since there's a default? (same below)

Suggested change

	* @param {boolean} as_pattern
	* @param {boolean} [as_pattern]