fix: support swc target for mjs

[haydnhkim]

If it is mjs, it is not included in the swcTarget even if I set it as an ES5 target.
I think the reason for the exclusion of mjs is that if the browser supports import, it is guaranteed to run ES6 or later code.
However, if you use a process like the one below, you'll run into problems.

• generate cjs, mjs code for target: es5 with tsup and publish it as a package named foo.
• a project named bar installs and uses the foo package. This project is set "type": "module".
• the bar project has imported the foo package.
• the bar project runs with ESM, it reads foo/index.mjs.
• when we build the bar project, the foo package is in node_modules, so it won't run in older browsers unless we transpile it further.

In addition to this issue, I think it's also a problem that tsup's target is set to ES5 and it doesn't transpile as expected.
So I've included MJS as well to solve this problem.

closes #1080

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Name	Status	Preview	Comments	Updated (UTC)
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[haydnhkim]

@sxzz Could you please review this PR?

[haydnhkim]

Thank you for your review!

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🎉 This PR is included in version 8.0.2 🎉

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