Improve support for "Platforms:" in LID file

[cgay]

This would avoid having to list all the platforms and would allow us to write the registry entry in the registry/generic/. It's also nice for self-documenting purposes compared to "here's a list of platforms; you decide if it's complete".

[cgay]

We could also use a way to indicate that a LID doesn't support a specific platform. For example we have a lot of libraries that only support x86-win32 and rather than listing all the other platforms in the non-x86-win32 LID file we could use one of these:

Platforms: -x86-win32

Antiplatforms: x86-win32

Platforms: not x86-win32

The specific syntax probably doesn't matter too much.

[cgay]

I would like to add support for these special platforms keywords:

• all -- like it says on the tin. This allows writing the registry entry in registry/generic
• macos -- currently would be equivalent to x86_64-darwin
• windows -- currently would be equivalent to win32
• unix -- currently would be equivalent to x86_64-darwin + x86_64-linux + x86_64-freebsd + x86_64-netbsd + all the 32-bit versions of those unixen.

When this is done, or even partly done, update the documentation here: https://opendylan.org/library-reference/lid.html#standard-lid-keywords

[cgay]

Also, would it make sense to write a registry file for each covered platform if the spec is not Platforms: all? That is, if it's Platforms: unix, write a registry file to registry/x86_64-linux, registry/x86_64-darwin, etc.

Currently deft only writes a registry file for the current platform, in the platform-specific registry directory.