* refactor(shared): extract `vim._list_insert` and `vim._list_remove`
* feat(lsp): add `vim.lsp.foldexpr()`
* docs(lsp): add a todo for state management
* feat(lsp): add `vim.lsp.folding_range.foldclose()`
* feat(lsp): schedule `foldclose()` if the buffer is not up-to-date
* feat(lsp): add `vim.lsp.foldtext()`
* feat(lsp): support multiple folding range providers
* refactor(lsp): expose all folding related functions under `vim.lsp.*`
* perf(lsp): add `lsp.MultiHandler` for do `foldupdate()` only once
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