* feat(ElTreeSelect): add tree select base component
* refactor(ElTreeSelect): use render function and move select/tree props to them self module
* fix(ElTreeSelect): init value not checked
* fix(ElTreeSelect): `toArray` ignores valid values
* fix(ElTreeSelect): expose not working when defined on mounted
* fix(ElTreeSelect): watch `modelValue` deep
* test(ElTreeSelect): add base unit test
* perf(ElTreeSelect): default slot should be a function
* fix(ElTreeSelect): `onNodeClick` can not call,
* test(ElTreeSelect): update unit test
* fix(ElTreeSelect): `onNodeClick` can not call,
* fix(ElTreeSelect): remove folder node when `checkStrictly` is false
* feat(ElTreeSelect): export `ElTreeSelect`
* fix(ElTreeSelect): `filterMethod` conflicts with `filterNodeMethod`
* docs(ElTreeSelect): add component docs
* fix(ElTreeSelect): fix lint
* docs(ElTreeSelect): fix lazy loading requires non-leaf nodes, and change mock labels
* docs(ElTreeSelect): the link address of the attributes is incorrect
* docs(ElTreeSelect): `dropdown` doesn't need the `-` symbol
* refactor(ElTreeSelect): use alias path and make sure vue is above to components
* refactor(ElTreeSelect): use a unified namespace for styles
* docs(ElTreeSelect): change option labels in default slots
* refactor(ElTreeSelect): import `ElOption` using unified entry and change the way to override the select click event
* style(ElTreeSelect): sort imports
* docs(ElTreeSelect): update the documentation for special codes
* refactor(ElTreeSelect): keep it consistent with the select style
* refactor(ElTreeSelect): use `isFunction` from `@element-plus/utils`
* refactor(ElTreeSelect): use single closing tag when no subset
* docs(ElTreeSelect): set `TreeSelect` promotion as `2.1.8`
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