Add full stub for Windows signals

[HertzDevil]

Resolves #7339.

This defines the 6 signals required by the C standard, using their values in MSVC's headers, plus one Windows-exclusive signal, Signal::BREAK. The old Signal::KILL no longer exists, as it was only a placeholder value needed by the standard library specs to compile, and this PR removes that need. (Most POSIX signals do not exist on Windows, but technically this was already a problem for PWR, STKFLT, and UNUSED on non-Linux systems.)

All of Signal's methods (#trap, #ignore, #reset) simply raise NotImplementedError. This is intentional, as the other platform-independent replacement APIs shall suffice for the moment.

[straight-shoota]

Looks good. Especially moving everything into Crystal::System. 👍
I only have a couple of comments on improving the wording in docs.

[HertzDevil]

Pinging @lbguilherme to see if this move into Crystal::System would affect WebAssembly in any way (probably not, as the things in src/kernel.cr are still guarded and the default prelude still doesn't require "signal")

[lbguilherme]

Pinging @lbguilherme to see if this move into Crystal::System would affect WebAssembly in any way (probably not, as the things in src/kernel.cr are still guarded and the default prelude still doesn't require "signal")

It seems fine as is, shouldn't affect anything. Actually, with your changes it could be possible to require signal on the prelude for wasm... I can test that later.