[vcpkg] Further JSON error improvements

[ras0219]

This PR does a few things:

1. Internally splits the representation (json.h) and parser (jsonreader.h) to reduce unintended dependencies across the codebase. The same refactor is also applied to Configuration/ConfigurationDeserializer.
2. Introduces levenshtein-distance suggestions for misspelled fields.
3. Minor tweak to the identifier error message.

Potential future work includes applying this levenshtein-distance calculation to search results.

As a drive by, this PR documents that comment fields ($xyz) are not supported in objects being used as sets with user-defined keys; they are only allowed in "struct-like" objects with a defined keyspace.

Example 1

{ "name": "Abseil", ... }

PS C:\src\vcpkg\testing> ..\vcpkg-old.exe install --feature-flags=manifests,registries --dry-run
Errors occurred while parsing C:\src\vcpkg\testing\vcpkg.json
    $.name: mismatched type: expected an identifier
See https://github.com/Microsoft/vcpkg/tree/master/docs/specifications/manifests.md for more information.

PS C:\src\vcpkg\testing> ..\vcpkg.exe install --feature-flags=manifests,registries --dry-run
Errors occurred while parsing C:\src\vcpkg\testing\vcpkg.json
    $.name (an identifier): must be lowercase alphanumeric+hyphens and not reserved
See https://github.com/Microsoft/vcpkg/tree/master/docs/specifications/manifests.md for more information.

Example 2

{ .., "depednecies": ["foo"], ... }

PS C:\src\vcpkg\testing> ..\vcpkg-old.exe install --feature-flags=manifests,registries --dry-run
Errors occurred while parsing C:\src\vcpkg\testing\vcpkg.json
    $ (a manifest): unexpected field 'depednecies'
See https://github.com/Microsoft/vcpkg/tree/master/docs/specifications/manifests.md for more information.

PS C:\src\vcpkg\testing> ..\vcpkg.exe install --feature-flags=manifests,registries --dry-run
Errors occurred while parsing C:\src\vcpkg\testing\vcpkg.json
    $ (a manifest): unexpected field 'depednecies', did you mean 'dependencies'?
See https://github.com/Microsoft/vcpkg/tree/master/docs/specifications/manifests.md for more information.

Example 3

{
  ...,
  "features": {
    "$abc" : true
  }
}

PS C:\src\vcpkg\testing> ..\vcpkg-old.exe install --feature-flags=manifests,registries --dry-run
Errors occurred while parsing C:\src\vcpkg\testing\vcpkg.json
    $.features (a features field): unexpected field '$abc'
See https://github.com/Microsoft/vcpkg/tree/master/docs/specifications/manifests.md for more information.

PS C:\src\vcpkg\testing> ..\vcpkg.exe install --feature-flags=manifests,registries --dry-run
Errors occurred while parsing C:\src\vcpkg\testing\vcpkg.json
    $.features (a set of features): unexpected field '$abc': must be lowercase alphanumeric+hyphens and not reserved
See https://github.com/Microsoft/vcpkg/tree/master/docs/specifications/manifests.md for more information.

[strega-nil]

Putting myself as a required reviewer.

[strega-nil]

Just a few things left :)