feat: improve let directive type

[ota-meshi]

Related to #394

This PR improves assigning the correct let directive type when the component type is available.

However, since @typescript-eslint/parser cannot be used to retrieve component types, users should use the experimental parser typescript-eslint-parser-for-extra-files instead. .
If the user is using @typescript-eslint/parser it will be the same as before.

[changeset-bot]

🦋 Changeset detected

Latest commit: 93373cb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte-eslint-parser	Minor

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[coveralls]

Pull Request Test Coverage Report for Build 5877142839

• 29 of 33 (87.88%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 90.517%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser/converts/attr.ts	25	29	86.21%

Totals
Change from base Build 5877064972:	-0.1%
Covered Lines:	2213
Relevant Lines:	2361

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💛 - Coveralls