[bug] support nulling out extended tsconfig.json properties

[mjames-c]

simple repro:

// tsconfig.base.json
{
  "compilerOptions": {
    "types": []
  },
}

// tsconfig.json
{
  "compilerOptions": {
    "types": null
  },
  "extends": "./tsconfig.base.json"
}

then run the below and note that compilerOptions.types is still set to the empty array, [].

$ built/local/tsgo -p path/to/tsconfig.json -showConfig
{
  ...
  "types": [],
  ...
}

[jakebailey]

This is a challenge in the Go code since we don't have two kinds of null anymore; I think we should have some sort of "unresolved" compiler options we use during load, which we then finalize into the final settings that don't need null.