Add Visual Studio formatters for String, Array, and Hash

[HertzDevil]

This patch provides Natvis formatter definitions for String, Array, and Hash:

Without the formatters:

System-wide Natvis files are only suitable for languages supported by Visual Studio itself, and Crystal isn't one of them, so the best way to use the formatters is by embedding them directly into the PDB file during a debug build:

> crystal build main.cr --debug --link-flags=/NATVIS:...\etc\msvc\crystal.natvis

The path to crystal.natvis must be an absolute path, because linking takes place inside the cache directory, not from the current working directory.

Perhaps eventually the compiler itself could be responsible for emitting the Natvis file itself? Related: #12112

[hugopl]

Perhaps eventually the compiler itself could be responsible for emitting the Natvis file itself

I think it should for windows debug builds with msvc, and it would be possible to disable it by a flag.