[vcpkg] Add initial versioning documentation

[ras0219]

This PR brings versioning into a near-ship state:

1. Adds reference documentation for the experimental user-facing features of versioning.
2. Renames $x-default-baseline to builtin-baseline and adds proper parsing support
3. Improves error messages throughout the versioning experience to help guide users to success
4. Improves performance of versioning to avoid all git manipulation in the happy path.
5. Uses git archive to achieve high performance git tree checkouts.
6. Improved test coverage of versioning
7. Users without any versioning fields will not experience any behavior changes with the versions feature flag enabled (+test).
8. Remove version=.

Two issues not addressed in this PR:

1. Emit warnings if the user uses overrides or constraints in a top-level manifest and doesn't have a baseline set (the user won't get versioning behavior in this case and that could be surprising)
2. Overrides currently uses schemed field names, however those schemes have no semantic meaning.

The initial spec is in #11758. Convenience link to the new docs is: https://github.com/ras0219/vcpkg/blob/dev/roschuma/versions/docs/users/versioning.md.

For local experiments, you will need to pull in versioning files:

git pull https://github.com/ras0219/vcpkg dev/roschuma/version-files

Example:

As an example, you can try:

{
    "name": "example",
    "version-date": "2020-01-01",
    "builtin-baseline": "334ba25e577b83331d5cfeb14d65768553ea1d9e",
    "dependencies": [
        {
            "$comment": "Note that this old rapidjson uses version-string",
            "name": "rapidjson",
            "version>=": "2019-06-28"
        },
        "zlib"
    ],
    "overrides": [
        {
            "name": "rapidjson",
            "version-string": "2019-06-28"
        }
    ]
}

and in that directory

vcpkg install --feature-flags=manifests,versions
vcpkg list --feature-flags=manifests,versions

Observing the result of 2019-06-28 and 1.2.11#9 for rapidjson and zlib respectively. If you didn't pull in the version files, you should see an error message like:

Failed at [/workspaces/vcpkg/toolsrc/src/vcpkg/registries.cpp(296)] with message:
Error: Couldn't find explicitly specified baseline `"334ba25e577b83331d5cfeb14d65768553ea1d9e"` in the baseline file, and there was no baseline at that commit or the commit didn't exist.
Error: while checking out baseline '334ba25e577b83331d5cfeb14d65768553ea1d9f:port_versions/baseline.json':
fatal: Path 'port_versions/baseline.json' does not exist in '334ba25e577b83331d5cfeb14d65768553ea1d9f'

This may be fixed by updating vcpkg to the latest master via `git pull`.
The current commit is "efe40c8c882c2bf3125b27e10294089f94e7b87b"

or

Failed at [/workspaces/vcpkg/toolsrc/src/vcpkg/install.cpp(877)] with message:
Error: no version entry for rapidjson at version 2019-06-28.
We are currently using the version in the ports tree (2020-09-14), since no rapidjson.json was found in /port_versions.

[ras0219]

I've added support for { "name": "zlib", "version>=": "1.2.11#8" } via the comment banners SUPPORT_HASH_SYNTAX; once the bikeshed has been painted, we can delete any offending code.