feat: support multiple build configs

[patzick]

closes #245

There are no breaking changes, and no action is required on the upgrade.

This change allows to have multiple build configurations in unbuild.config file. This is very much needed if you have different entry points and want separate flows for them or even a single entry point but a few different outcomes.

so basically you can do:

export default defineBuildConfig([
  {
    // ... first config
  },
  {
    // ... second config
  },
]);

or still do

export default defineBuildConfig({
  // ... single config like it is now
});

Short description of changes:

• beginning of the build command has been extracted to read multiple configurations. These configurations are merged the same way they were and run one after another, with the rest of the code moved to an internal singleConfigBuild build. This way API of the package is not changing
• hooks are merged so we can simplify their registration
• fixture example is updated to show a simple use case

[patzick]

hey @pi0 :) Could you take a look here?
It would be great if that could be released soon 🤗

[pi0]

Hi dear @patzick rest assured it is definitely on my todo list to be moved forward ASAP 👍🏼 Sorry the review took long.

[pi0]

Thanks 💯