[fix] allow users to override build target

[gunggmee]

This PR tries to fix #2565 while ensuring the overridden value still satisfies the constraints of the default build target. It contains following changes:

• Sets es2020 as default build target
• Allows users to override build target
• Ensures overridden build target satisfies the constraints of es2020 by injecting the default target into overridden target(s). This makes esbuild to check the constraints.

Made a draft PR since it doesn't satisfies some items in the checklist. Please help me with additional commits.

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Tests

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[changeset-bot]

🦋 Changeset detected

Latest commit: 0a19bfb

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[bluwy]

Nice. Thanks for implementing this! I probably wasn't clear in my comment on the issue, but my intention was that we shouldn't validate build.target and accept what the user gives as is. It's pretty cool that you have something working nonetheless. From a look I think there's a few more things we can do:

1. Handle targets like es5 and es6 (not in year format)
2. Report to the user that the build target setting changed.

But IMO I think it's more work than it's worth since there would always be edge cases where this behaviour is undesirable. So I'm in favour of removing the validation logic for now. Curious on what the other maintainers' thought on this.

Other than that, the last thing is to add a changset via pnpx changeset, and it should be good to go.

[gunggmee]

@bluwy Oh I see. Just moved server.build.target to default_config to make the build process don't warn on overriding. Also added the changeset.

[bluwy]

LGTM. Will wait for the others to check out before merging.