Problem:
1. Open a relatively large file (so the server needs some time to
process the request).
2. Then immediately execute `:bdelete`.
3. Once the request is completed, the handler will obtain the bufstate
of a buffer already unloaded.
Error executing vim.schedule lua callback: ...7841_1/share/nvim/runtime/lua/vim/lsp/_folding_range.lua:119: assertion failed!
stack traceback:
[C]: in function 'assert'
...7841_1/share/nvim/runtime/lua/vim/lsp/_folding_range.lua:119: in function 'multi_handler'
...7841_1/share/nvim/runtime/lua/vim/lsp/_folding_range.lua:140: in function 'handler'
...HEAD-c137841_1/share/nvim/runtime/lua/vim/lsp/client.lua:669: in function ''
vim/_editor.lua: in function <vim/_editor.lua:0>
Solution:
On detach, cancel all pending textDocument_foldingRange requests.
* 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
