Folds: rename query files to folds.scm to be consistent

We use plural names for all query files except folds.

CONTRIBUTING.md

@@ -215,7 +215,7 @@ Possible scope values are:
 
 ### Folds
 
-You can define folds for a given language by adding a `fold.scm` query :
+You can define folds for a given language by adding a `folds.scm` query :
 
 ```
 @fold

lua/nvim-treesitter/fold.lua

@@ -13,7 +13,7 @@ local folds_levels = utils.memoize_by_buf_tick(function(bufnr)
 
   local matches
   if query.has_fold(lang) then
-    matches = query.get_capture_matches(bufnr, "@fold", "fold")
+    matches = query.get_capture_matches(bufnr, "@fold", "folds")
   elseif query.has_locals(lang) then
     matches = query.get_capture_matches(bufnr, "@scope", "locals")
   else

lua/nvim-treesitter/query.lua

@@ -17,7 +17,7 @@ M.base_language_map = {
   tsx = {'typescript', 'javascript', 'jsx'},
 }
 
-M.built_in_query_groups = {'highlights', 'locals', 'textobjects', 'fold'}
+M.built_in_query_groups = {'highlights', 'locals', 'textobjects', 'folds'}
 
 -- Creates a function that checks whether a certain query exists
 -- for a specific language.