refactor(lsp): stateful data abstraction, vim.lsp.Capability #34639

Problem:
Closes #31453

Solution:
Introduce `vim.lsp.Capability`, which may serve as the base class for
all LSP features that require caching data. it
- was created if there is at least one client that supports the specific method;
- was destroyed if all clients that support the method were detached.

- Apply the refactor for `folding_range.lua` and `semantic_tokens.lua`.
- Show active features in :checkhealth.

Future:
I found that these features that are expected to be refactored by
`vim.lsp.Capability` have one characteristic in common: they all send
LSP requests once the document is modified. The following code is
different, but they are all for this purpose.

- semantic tokens:
fb8dba413f/runtime/lua/vim/lsp/semantic_tokens.lua (L192-L198)
- inlay hints, folding ranges, document color
fb8dba413f/runtime/lua/vim/lsp/inlay_hint.lua (L250-L266)

I think I can sum up this characteristic as the need to keep certain
data synchronized with the latest version computed by the server.
I believe we can handle this at the `vim.lsp.Capability` level, and
I think it will be very useful.

Therefore, my next step is to implement LSP request sending and data
synchronization on `vim.lsp.Capability`, rather than limiting it to the
current create/destroy data approach.

test/functional/plugin/lsp/folding_range_spec.lua

@@ -4,7 +4,6 @@ local Screen = require('test.functional.ui.screen')
 local t_lsp = require('test.functional.plugin.lsp.testutil')
 
 local eq = t.eq
-local tempname = t.tmpname
 
 local clear_notrace = t_lsp.clear_notrace
 local create_server_definition = t_lsp.create_server_definition
@@ -121,52 +120,6 @@ static int foldLevel(linenr_T lnum)
     api.nvim_exec_autocmds('VimLeavePre', { modeline = false })
   end)
 
-  describe('setup()', function()
-    ---@type integer
-    local bufnr_set_expr
-    ---@type integer
-    local bufnr_never_set_expr
-
-    local function buf_autocmd_num(bufnr_to_check)
-      return exec_lua(function()
-        return #vim.api.nvim_get_autocmds({ buffer = bufnr_to_check, event = 'LspNotify' })
-      end)
-    end
-
-    before_each(function()
-      command([[setlocal foldexpr=v:lua.vim.lsp.foldexpr()]])
-      exec_lua(function()
-        bufnr_set_expr = vim.api.nvim_create_buf(true, false)
-        vim.api.nvim_set_current_buf(bufnr_set_expr)
-      end)
-      insert(text)
-      command('write ' .. tempname(false))
-      command([[setlocal foldexpr=v:lua.vim.lsp.foldexpr()]])
-      exec_lua(function()
-        bufnr_never_set_expr = vim.api.nvim_create_buf(true, false)
-        vim.api.nvim_set_current_buf(bufnr_never_set_expr)
-      end)
-      insert(text)
-      api.nvim_win_set_buf(0, bufnr_set_expr)
-    end)
-
-    it('only create event hooks where foldexpr has been set', function()
-      eq(1, buf_autocmd_num(bufnr))
-      eq(1, buf_autocmd_num(bufnr_set_expr))
-      eq(0, buf_autocmd_num(bufnr_never_set_expr))
-    end)
-
-    it('does not create duplicate event hooks after reloaded', function()
-      command('edit')
-      eq(1, buf_autocmd_num(bufnr_set_expr))
-    end)
-
-    it('cleans up event hooks when buffer is unloaded', function()
-      command('bdelete')
-      eq(0, buf_autocmd_num(bufnr_set_expr))
-    end)
-  end)
-
   describe('expr()', function()
     --- @type test.functional.ui.screen
     local screen
@@ -182,6 +135,29 @@ static int foldLevel(linenr_T lnum)
       command([[split]])
     end)
 
+    it('controls the value of `b:_lsp_folding_range_enabled`', function()
+      eq(
+        true,
+        exec_lua(function()
+          return vim.b._lsp_folding_range_enabled
+        end)
+      )
+      command [[setlocal foldexpr=]]
+      eq(
+        nil,
+        exec_lua(function()
+          return vim.b._lsp_folding_range_enabled
+        end)
+      )
+      command([[set foldexpr=v:lua.vim.lsp.foldexpr()]])
+      eq(
+        true,
+        exec_lua(function()
+          return vim.b._lsp_folding_range_enabled
+        end)
+      )
+    end)
+
     it('can compute fold levels', function()
       ---@type table<integer, string>
       local foldlevels = {}