feat(treesitter)!: don't parse tree in get_parser() or start()

**Problem:** `vim.treesitter.get_parser()` and `vim.treesitter.start()`
both parse the tree before returning it. This is problematic because if
this is a sync parse, it will stall the editor on large files. If it is
an async parse, the functions return stale trees.

**Solution:** Remove this parsing side effect and leave it to the user
to parse the returned trees, either synchronously or asynchronously.

runtime/doc/news.txt

@@ -158,6 +158,11 @@ TREESITTER
   if no languages are explicitly registered.
 • |vim.treesitter.language.add()| returns `true` if a parser was loaded
   successfully and `nil,errmsg` otherwise instead of throwing an error.
+• |vim.treesitter.get_parser()| and |vim.treesitter.start()| no longer parse
+  the tree before returning. Scripts must call |LanguageTree:parse()| explicitly. >lua
+    local p = vim.treesitter.get_parser(0, 'c')
+    p:parse()
+<
 
 TUI
 

runtime/lua/vim/treesitter.lua

@@ -61,8 +61,6 @@ function M._create_parser(bufnr, lang, opts)
     { on_bytes = bytes_cb, on_detach = detach_cb, on_reload = reload_cb, preview = true }
   )
 
-  self:parse(nil, function() end)
-
   return self
 end
 

runtime/lua/vim/treesitter/highlighter.lua

@@ -148,8 +148,6 @@ function TSHighlighter.new(tree, opts)
     vim.opt_local.spelloptions:append('noplainbuffer')
   end)
 
-  self.tree:parse(nil, function() end)
-
   return self
 end