Loosen extension filtering to cover more possibilities

[vnphanquang]

Today, as of mdsvex@0.11.0, if .md.svelte is provided to the extension / extensions config, MDsveX will take it as .svelte, effectively making it process all .svelte files, which is not ideal.

This PR addresses this issue by making the mapping more relaxed (please see change).

The rationale for using .md.svelte is to take advantage of the default editor support for .svelte files, while still marking them as markdown-related.

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If this issue has come up before and the proposed change is not the direction MDsveX wants to take, I apologize and will respect decisions made by code owner.

Thank you

[changeset-bot]

🦋 Changeset detected

Latest commit: c930db9

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

This PR includes changesets to release 1 package

Name	Type
mdsvex	Patch

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