Add constants for staticlib to `std::env::const`

[ranger-ross]

We have prefix/suffixes for OS specific extensions in std::env::const for bins and dynamic libs.
But we do not have a constant for staticlibs.

I think it would be helpful to include consts for staticlibs as well. (especially since they are different windows MinGW & windows MSVC)

Proposed name:

• std::env::const::STATICLIB_SUFFIX
• std::env::const::STATICLIB_EXT

[ranger-ross]

@rustbot label +T-lib-api +C-feature-request

[jieyouxu]

This should probably be proposed as an ACP against https://github.com/rust-lang/libs-team/issues/