Use unbuild for build system

[Holi0317]

Fixes #63.

Import of mimetext has been broken since v3.0.19 in both CJS and ESM node. Bundler environment is still working.

arethetypeswrong can check and verify this:

While the entrypoint are correctly advertised as cjs/esm and they are using correct require/import statements, all entrypoints are trying to require/import .js file and it breaks cjs imports. Because the import are not rewritten with correct path.

This PR replaces the rollup+babel build system with unbuild. There might be some breaking changes introduced by this:

• Minimum node version set to 18 (current supported version) just to set a baseline. This was never mentioned anywhere in the repo before
• Babel and corejs are removed. .browserslistrc, while still in the repo, was no longer read by build system. The browser build probably won't run on IE10
• Change build directory from build to dist because that's unbuild's convention. If this is a bit annoying we can write a config for unbuild to change that

[muratgozel]

just heard about this tool, looks better than tsup. i will give it a try, thank you @Holi0317

plus, there is also pkgroll

[Holi0317]

Yeah I also saw pkgroll exist.

Now that you've fixed the packaging, do you want to move forward with unbuild, pkgroll, tsup or keep the packaging infrastructure as is?

I don't think I should be making any big decision here but I am more than happy to experiment with different tools and see which fits.

[muratgozel]

i prefer to trust tsup for now as its more mature compared to others so gonna leave the packaging as it is. i will use the bundler we gonna choose here in my other packages as well so i want to be sure its good enough.

i will give both unbuild and pkgroll a try in one of my other packages.

are you able to compare any of these three bundlers? i mean have you used them?