Change UI so that combined grammars have only one tab

We keep getting users that get confused with the antlr-lab UI. https://stackoverflow.com/questions/78080471/using-spaces-as-delimiters-in-antlr4 . People paste a combined grammar into the "parser" text box, but then fail to clear the "lexer" text box. The UI needs to differentiate between combined- and split-grammars.

Instead, provide a check box "combined" that is "on" by default. When it is "on", only show a "parser" grammar. Do not provide a "lexer" tab/text box.

Start by locating the antlr-lab UI code that renders the parser and lexer text boxes, then run the app to reproduce the combined-grammar confusion. Add the default-on “combined” option so only the parser input is shown when enabled, while split grammars retain both inputs. Done means both modes visibly provide the intended grammar inputs.

Written by the indexing model from the issue text.