[antlr4 grammar/CI build] Fix for 2609

[kaby76]

This PR is for the antlr/antlr4/ grammar #2609. There's also a change to the build, but that is only for supporting Cpp split grammars with base class code that needs to be compiled and linked.

• Updated the antlr/antlr4 grammar with the new 4.10.1 syntax for caseInsensitive.
• Synced the base class code between targets. The targets that are implemented are Cpp, CSharp, Java, JavaScript, and Python3; I did not implement any new targets.
• Added the antlr/antlr4/ grammar to CI testing to help prevent targets from becoming out of sync.
• Note, forking of the antlr/antlr4/ grammar was required for Cpp and Python3. This is because there really is no such thing as "target agnostic format", but what i call "pseudo target agnostic format". It's a syntax that is identical for most, but not all, targets. We can get "target agnostic format", but only if we introduce a syntax for "self", e.g., $parser and $lexer handles with a '.' reference. Cpp will also need to generate a #include for the base class, which the Antlr4 tool does not do.
• trgen needed to be updated to fix a bug in the reference to the "additional_sources" attribute used in the CMakeLists.txt templated file.
• There is a difference between this grammar and the one in the Intellij Antlr plugin in lexing options, but I haven't resolved that difference. Instead, I just added a "TODO".

[kaby76]

• No short cuts--I need to update trgen to add all source files in the Cpp/ to the generated CMakeLists.txt file, and remove antlr/antlr4/Cpp/CMakeLists.txt that only works on my system.

[teverett]

@kaby76 thanks