feat: add defaultUnocssConfig to webpack plugin and unocss vscode extension

[TrickyPi]

close #1032

also make webpack plugin use @unocss/preset-uno as default preset.

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[antfu]

Well, it's intentional to have @unocss/vite and @unocss/webpack preset irrelevant so higher integrations that provide their own presets could reuse them if they want. I am ok to have the default theme on the extension but not the webpack plugin. Thanks

[TrickyPi]

The default presets will be overridden by custom configuration, like this describition https://github.com/unocss/unocss/blob/main/README.md?plain=1#L166, so i think there will be no issues about integration(maybe i'm wrong).
perhaps we can export a function like UnocssVitePlugin for webpack user,

[antfu]

I don't want to force download the presets for those who don't need them. Maybe we could have unocss/webpack which binds the default preset and ask @unocss/webpack as peerDeps

[TrickyPi]

if we ask @unocss/webpack as peerDeps, maybe we need list this info in instruction. Could we add instruction about how to use unocss on webpack to README.md?

[antfu]

Sure, I would handle the docs part

[TrickyPi]

Thanks for your commits.