feat(treesitter): add folding for InspectTree (#27518)

As the InspectTree buffer is now a valid tree-sitter query tree, we can
use the bundled fold queries to have folding for the tree.

runtime/lua/vim/treesitter.lua

@@ -461,7 +461,8 @@ end
 ---
 --- While in the window, press "a" to toggle display of anonymous nodes, "I" to toggle the
 --- display of the source language of each node, "o" to toggle the query editor, and press
---- <Enter> to jump to the node under the cursor in the source buffer.
+--- <Enter> to jump to the node under the cursor in the source buffer. Folding also works
+--- (try |zo|, |zc|, etc.).
 ---
 --- Can also be shown with `:InspectTree`. *:InspectTree*
 ---

runtime/lua/vim/treesitter/dev.lua

@@ -159,7 +159,10 @@ end
 local function set_dev_properties(w, b)
   vim.wo[w].scrolloff = 5
   vim.wo[w].wrap = false
-  vim.wo[w].foldmethod = 'manual' -- disable folding
+  vim.wo[w].foldmethod = 'expr'
+  vim.wo[w].foldexpr = 'v:lua.vim.treesitter.foldexpr()' -- explicitly set foldexpr
+  vim.wo[w].foldenable = false -- Don't fold on first open InspectTree
+  vim.wo[w].foldlevel = 99
   vim.bo[b].buflisted = false
   vim.bo[b].buftype = 'nofile'
   vim.bo[b].bufhidden = 'wipe'