Scan nested fields properly

* yasnippet.el (yas--scan-for-field-end): New function.
(yas--field-parse-create): Use it instead of yas--scan-sexps, which
isn't able to distinguish between ${...} (a nested field) and
{...} (plain old braces in the snippet text).
* yasnippet-tests.el (yas-escaping-close-brace): New test.

yasnippet-tests.el

@@ -947,6 +947,18 @@ mapconcat #'(lambda (arg)
       (yas-mock-insert "baz")
       (should (string= (yas--buffer-contents) "foobaaarbazok")))))
 
+(ert-deftest yas-escaping-close-brace ()
+  "Close braces may be escaped with braces, reduction from eglot issue.
+See https://github.com/joaotavora/eglot/issues/336."
+  (with-temp-buffer
+    (yas-minor-mode +1)
+    ;; NOTE: put a period at the end to avoid the bug tested by
+    ;; `protection-overlay-no-cheating'.
+    (yas-expand-snippet "${1:one{\\}}, ${2:two{\\}}.")
+    (yas-next-field)
+    (yas-next-field)
+    (should (string= (buffer-string) "one{}, two{}."))))
+
 
 ;;; Misc tests
 ;;;

yasnippet.el

@@ -4719,6 +4719,13 @@ SAVED-QUOTES is the in format returned by `yas--save-backquotes'."
           yas--indent-markers))
   (setq yas--indent-markers (nreverse yas--indent-markers)))
 
+(defun yas--scan-for-field-end ()
+  (while (progn (re-search-forward "\\${\\|}")
+                (when (eq (char-before) ?\{)
+                  ;; Nested field.
+                  (yas--scan-for-field-end))))
+  (point))
+
 (defun yas--field-parse-create (snippet &optional parent-field)
   "Parse most field expressions in SNIPPET, except for the simple one \"$n\".
 
@@ -4735,7 +4742,9 @@ When multiple expressions are found, only the last one counts."
   ;;
   (save-excursion
     (while (re-search-forward yas--field-regexp nil t)
-      (let* ((brace-scan (yas--scan-sexps (1+ (match-beginning 0)) 1))
+      (let* ((brace-scan (save-match-data
+                           (goto-char (match-beginning 2))
+                           (yas--scan-for-field-end)))
              ;; if the `brace-scan' didn't reach a brace, we have a
              ;; snippet with invalid escaping, probably a closing
              ;; brace escaped with two backslashes (github#979). But