Refactoring of target properties and their validation

[lhstrh]

This PR reworks the way we deal with target properties. It makes them easier to specify, manipulate, and verify. It cuts down on the use of lambda functions, which makes the code a bit more verbose, but also more reusable and more readable. The TargetProperty class that had become a depository of random helper classes has been cleaned and brought down from 1800 LOC to less than 300.

While the goal of this refactoring is to retain the same functionality, some changes were made in the name of enforcing consistency:

• --target-compiler CLI argument changed to --compiler so that it matches the compiler target property
• error reporting changed to report more precisely (on the Element instead of KeyValuePair) and with simpler messages
• Unrecognized platform causes NPE #1919
• Stateless target properties #2039
• let TypeScript backend honor --no-compile flag/target property
• let Rust and C++ use the same build type (Debug)
• register target properties in Target class and revise "supported by" mechanism
• add method for parsing from JSON (for interop with lingo)
• base CLI args directly on target property names
• clear separation between configurators and transformers in the test framework (c96a794)
• add comments

Fixes #1558.
Fixes #2001.
Fixes #1919.
Fixes #1859.

[cmnrd]

This is a great improvement! I haven't looked carefully at all the changes yet, but I left some comments in places where I noticed issues or had questions.

[lsk567]

This is a big effort that makes target properties more robust by relying less on String operations. Looks good to me overall. Thanks for working on this @lhstrh. :-)