Fix Metafile types to account for non-JS assets

[iamakulov]

In the metafile output, some non-JS output assets don’t have imports and/or exports fields in the metafile. This change fixes the TS types accordingly.

[iamakulov]

I’m not sure where the source code for esbuild.github.io is, so I didn’t update it – but I believe https://esbuild.github.io/api/#metafile will also need to be changed accordingly.

[evanw]

Thanks for the PR. However, I think the metafile would be easier to use if these properties were just always present. So I'm going to fix this problem that way instead. Then you shouldn't have to check for undefined in these places.