Add compiler customization schema

[Pennycook]

Related issues

• Part of Support custom compiler arguments #145; the modified schema allows a configuration file to define custom implicit compiler behavior (i.e., macro definitions, include paths and include files) based on compiler arguments.

Proposed changes

• Extend the schema to support:
  • alias_of: Defines that a compiler is an alias of another (e.g., icpx is an alias of icx)
  • [[compiler."compiler name".parser]]: A table of arguments for argparse.
  • [[compiler."compiler name".modes]]: A table of "modes", each of which is a collection of compiler behaviors that can later be enabled by a flag (e.g., -fopenmp enables the "openmp" mode, which in turn defines _OPENMP, etc).
  • [[compiler."compiler name".passes]]: A table of "passes", each of which is a collection of compiler behaviors associated with a separate pass over a source file (e.g., -fsycl-targets=spir64,nvidia-ptx-cuda will compile the source file to SPIR-V and PTX, with different macros defined).
• Add new tests for the proposed schema.

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I appreciate that JSON schema can be really difficult to read, so I've tried extra hard to improve the readability of the tests here. Each call to _test_toml_string without an assertRaises demonstrates the syntax of a valid use-case. Each call with an assertRaises is intended to test either a oneOf, additionalProperties or required clause in the schema.

[Pennycook]

While working on fixing a bug in a later stage of this, I realized that some of the compiler definition files I'd written were failing validation. I've added the missing keys to the schema.