fix(esbuild): set js loader for build transpile

[bluwy]

Description

fix #10147

Since renderChunk should always only deal with JS files, we can set loader: 'js' as an optimization, and also fix #10147 so it doesn't have to detect the extension manually.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

We'll be accepting not having a .js extension for chunks as a feature after merging this PR, and maybe we should keep it more strict. I like this as an optimization though. We aren't documenting a change so lets merge the PR, we could later still decide to force a js extension if we have good reasons to do it.