fix: handle .htm as same as .html (fix #10997)

[Potato1682]

Description

Fixes #10997.
This will add support parsing .htm file as same as .html.
When there are both index.html and index.html, only index.html is loaded.

Additional context

I changed the .html file extension conditions in various files, so I might have changed some extreme conditions. Can you confirm that these changes are correct?
Also, according to #10997, it has unexported htmlLangRE and found isHTMLRequest(request: string): boolean to test with the regex, but isHTMLRequest is only used in /packages/vite/src/node/plugins/define.ts:110-118 and has the normalized name like other functions. Should we use it to check the extension of HTML files?

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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