[postgresql] Fix for #4324; add ports.

[kaby76]

This is a fix for #4324. The grammar was not "target agnostic" and there were just two targets implemented. With this change almost all targets are supported. I didn't bother with the implementation of the PHP port because it is very slow in general and contains errors in ATN Config hash value and equality computions. This problem becomes accute with ambiguity and fallbacks. I also didn't write the Swift port because I don't develop on MacOS.

Target Agnostic is a specific coding style for .g4 files to allow any target to be generated from one copy of the .g4 files: All @members and @headers are replaced with a comment; Actions { ... } and { ... }?contain only parameterless method calls with a this. prefix; transformGrammar.py is used to modify the .g4s just prior calling the Antlr Tool specific for the target because there are still minor variations for targets from the established format. The purpose of Target Agnostic is to avoid the major problem of keeping multiple copies of the .g4 files in synch across ports.

The following ports were added: Antlr4ng, Cpp, Dart, Go, JavaScript, Python3, TypeScript. Existing ports are: CSharp, Java. Note, the Go port became out of synch because it was not plugged into the build. It now works.

Notes

• The spaces on either side of the method call in an action have been removed. Python requires code to begin in very specific columns in order to nest within an if-statement.
• The readme has been updated with a description of how to use transformGrammars.py.
• All the methods in actions have been updated.
• There is change in one of the rules from [postgresql] Fixes for #4321, #4322 #4323. This "disappears" when performing the merge after the other PR.
• I'm unclear why there are lexer rules that match empty. I had to remove the empty curly actions { } in these rules because the code does not compile for Python3.

[kaby76]

(Requires #4323 to be merged first.)

[kaby76]

(Finally a clean build. Failures had nothing to do with this PR. It's all a matter of chance. See #4326.)