fix(formatexpr): does not fallback to the built-in formatexpr (#238)

This makes the behavior of formatexpr more consistent with
`vim.lsp.formatexpr`; do not run vim's builtin formatter.

Problem: When conform's formatexpr is used and (range or buffer) but
conform can't do formatting and there is no LSP formatter with the
formatting capabilities, it will fall back to the (wrong) built-in
formatting, which might result in simply concatenating all the words.

This is a breaking change, reverting the behavior introduced in #55.

lua/conform/init.lua

@@ -719,7 +719,8 @@ M.formatexpr = function(opts)
     -- No formatters were available; fall back to lsp formatter
     return vim.lsp.formatexpr({ timeout_ms = opts.timeout_ms })
   else
-    return 1
+    -- Do not fallback to built-in formatter.
+    return 0
   end
 end