feat: introduce syncMode to control the behavior of generating files

[kevinmarrec]

Description

Introduce the syncMode option to control the behavior of generating files.

Default to default

• append: only append the new components to the existing files.
• overwrite: overwrite the whole existing files with the current components.
• default: use append strategy when using dev server, overwrite strategy when using build.

Linked Issues

Resolves #804 (Please confirm ⚠️)

Linked Pull Requests

Closes #844

Additional context

Current logic is removeUnused === !this._server, which means only remove unused components when building and is dependent on vite mode ("Is there a dev server ?").

To prevent any breaking change, a default mode is required to follow current behavior, while append mode will always append which means also append with vite build.

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[xiaweiss]

@kevinmarrec I found a bug

doc say default: use append strategy when using dev server, overwrite strategy when using build.

But in reality, the overwrite was executed when using dev server.

[xiaweiss]

@antfu 合并请求里，没有对 syncMode: 'default' 选项做任何处理，只是不等于 'append' 就默认执行了 'overwrite' 的效果。

导致在启动 dev-server 开发时，运行了一个页面，会把其他页面的 icon 自动导入全部删除。

 setupViteServer(server: ViteDevServer) {
    if (this._server === server)
      return

    this._server = server
    this._removeUnused = this.options.syncMode !== 'append'
    this.setupWatcher(server.watcher)
  }