feat: auto-import for composables

[antfu]

Close nuxt/nuxt#11862

Continue #969

Followup nuxt/nuxt#12789

• Update docs
• Add new autoImports:dirs hook and autoImports: { dirs } option
• Auto scan ~/composables with both default exports and named exports (using static ESM syntax analyzes)

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[antfu]

@pi0 Our current implementation does not expect the auto-import map to be changed during the runtime, I will need to refactor some parts and regenerate the match regex upon changes

[pi0]

What about generating a virtual #imports entry? This way we can simply transform implementation to just update regex and can also use same artifact when transform or types not available (while we can shim with #imports)

[pi0]

Followup nuxt/nuxt#12789