feat: support importing json modules

[kricsleo]

resolves #332
resolves #184

(Can also resolve some downstream issues: unjs/unbuild#430)

[kricsleo]

This is so strange that the CI test has been returning inconsistent results for testcases/import-json.
Sometimes the result is declare let name: ... and sometimes it's declare const name: ....
I don't have this issue locally.

[Swatinem]

The testsuite runs twice, once with the "latest" (aka the declared dependency version of the typescript compiler, which might need a bump in either case).
And then a second time with the "minimum supported TS version", which is 4.5.

That second run fails, whereas the first one succeeds. Some somewhere in between the TS compiler itself changed how it generates declarations for JSON.

You can add the minimum TS version the test should run with in the meta file. Feel free to just bump that to the latest version, otherwise you can also bisect the specific TS version that did this change, but thats probably not worth the effort.

[kricsleo]

Thanks for the detailed explanation ❤️
I tried to bisect and determine that this behaviour was caused by microsoft/TypeScript#53206.

The impacted official version is v5.1.3.

• < v5.1.3(The closest version: 5.0.4)： declare const name: ....
• >= v5.1.3： declare let name: ....

I'm not sure if it's meant to change the behavior of JSON modules, but since it doesn't impact actual usage, I'll let it be. 👌🏻