Add proposal for #feature directive

[tangent-vector]

This proposal is a companion to SP#012.

[csyonghe]

I am worried that supporting individual feature flags can lead to fragmentation of possible compiler modes that we need to test and maintain support for.

Can we simply have something simpler with versions, by having #language slang 2025.1 stable and #language slang 2025.1 experimental. We are already suffering from the divergent behavior for some existing compiler flags such as -allow-glsl, and I want to be cautious to not make this situation worse.