Merge pull request #15960 from overleaf/mj-bibtex-field-plus

[lezer] Support plus character in BibTeX field name

GitOrigin-RevId: a8af42fa9c372b36ed2ec0354bafa76154dd8d18

services/web/frontend/js/features/source-editor/lezer-bibtex/bibtex.grammar

@@ -7,7 +7,7 @@
   whiteSpace { $[\t\n ]+ }
   Identifier { ![,{} \t\n%]+ }
   StringName { $[a-zA-Z:_] $[a-zA-Z:_0-9-]* }
-  FieldName {$[a-zA-Z-_0-9]+}
+  FieldName {$[a-zA-Z-_0-9+]+}
   LiteralString {
     '"' (!["] | "\\" _)* '"'?
   }