feat(rsc): ability to merge client reference chunks

[hi-ogawa]

Description

• Closes Support merging client reference chunks #605

todo

• test
• doc
• group based on rsc async chunk (follow up) feat(rsc): ability to merge client reference chunks based on server chunk usage #767

idea

Example:

Use clientChunks to specify custom grouping custom-a = mod1, mod2 and custom-b = mod3

plugins: [
  rsc({
    clientChunks: (id) => {
      if (id === 'mod1' || id === 'mod2') return 'custom-a'
      if (id === 'mod3') return 'custom-b'
    }
  })
]

Internally virtual modules will look like following:

// virtual:vite-rsc/client-references
// (the shape of this virtual is same as before)
export default {
  mod1: async () => {
    const m = await import("virtual:vite-rsc/client-reference/group/custom-a");
    return m.export_mod1;
  },
  mod2: () => {
    const m = await import("virtual:vite-rsc/client-reference/group/custom-a");
    return m.export_mod2;     
  },
  mod3: () => {
    const m = await import("virtual:vite-rsc/client-reference/group/custom-b");
    return m.export_mod3; 
  }
}

// virtual:vite-rsc/client-reference/group/custom-a
import * as import_mod1 from "(mod1)";
import * as import_mod2 from "(mod2)";
export const export_mod1 = { 
  // filter by `renderedExports` to tree shake unued client components
  // (this is a same logic as before)
  Counter: import_mod1.Counter,
  ...
}
export const export_mod2 = { ... import_mod2 ... }

// virtual:vite-rsc/client-reference/group/custom-b
import * as import_mod3 from "(mod3)";
export const export_mod3 = { ... import_mod3 ... }

[hi-ogawa]

Actually it might be no-brainer to just go with #767 as a default behavior.

[hi-ogawa]

We should do so at some point, but for now let's merge this.