Change UI so that combined grammars have only one tab

[kaby76]

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.

[ericvergnaud]

How about going a step further and showing "combined" when the check box is on vs "parser" and "lexer" when it's off ?

[ericvergnaud]

And how about auto detection ?

[kaby76]

All good ideas, any of which would help make it work for the better.