fix(issue:4258) variable interpolation after math

[puckowski]

What:

Fixes variable interpolation issue after previous variable addition.

Why:

Current Less.js takes:

@a: 1px;
@b: 2px;
@c: @a + @b;

@radio-cls: radio;
@radio-cls-checked: @{radio-cls}_checked;

.@{radio-cls-checked} {
  border-color: #fff;
}

and produces:

._checked {
  border-color: #fff;
}

With fix the CSS is:

.radio_checked {
  border-color: #fff;
}

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

Tests passing locally. Updated tests for variables.

[puckowski]

I submitted a few PRs. I expect rebase(s) will be needed in order to merge all of them, so let me know when a rebase is needed.