feat!: use .mjs for esm output

[bluwy]

When using this in node, the CJS output is being loaded even though the library exports ESM output. I've adjusted the ESM output to end with the .mjs extension, and added an exports field so that node resolves to it.

[antfu]

This would be a breaking change. Giving magic-string currently support down to Node v10, we might need to introduce a breaking change to deprecate v10 to support this.

[bluwy]

Good point. To be safe, I agree this should be a minor bump for 0.x. Though I think normal Node 10 resolution algorithm should avoid the .mjs paths, since Node 10 doesn't read pkg.module and pkg.exports, so we could get away bumping the node version perhaps 🤔