Dev tool: IE11-only SCSS mixin

[mikemai2awesome]

Jira

N/A

Summary

Adds a SCSS mixin for Bolt devs to define IE11 CSS.

Details

Created bolt-ie11-only mixin under tools in the core. This will make it easier every time we need to write CSS that only applies to IE11.

How to test

1. Run the branch locally.
2. Add the following test code to any component CSS to target that component:

@include bolt-ie11-only {
  * {
    background-color: red !important;
  }
}

3. Make sure whatever you targeted turned to red.

[sghoweri]

I'm fine with the idea of this BUT I want to make sure we're only using this mixin when we physically can't target based on feature support of some specific CSS property or style something via progressive enhancement (ie. no @support styles initially + @support styles on top of that)