Replace importcfg.json with config TOML file

[Pennycook]

Adopting Python 3.12 means that we have access to tomllib, and can replace a JSON configuration file like this:

{
  "compilers": [
    {
      "name": "nvcc",
      "options": [
        "-D __CUDACC_VER_MAJOR__=11",
        "-I /path/to/cuda/installation",
      ]
    },
    {
      "name": "gcc",
      "options": [
        "-D__GNUC__=12",
      ]
    },
  ]
}

With a much more user-friendly TOML file:

[compiler.nvcc]
options = [
  "-D__CUDACC_VER_MAJOR__=11",
  "-I /path/to/cuda/installation",
]
 
[compiler.gcc]
options = [
  "-D__GNUC__=12",
]

Related issues

Replaces functionality originally added by #27.

Proposed changes

• Replace cbi/importcfg.json with cbi/config (assumed TOML).
• Add functions to validate TOML against a schema for the equivalent JSON.
• Add tests for the new TOML validation.