fix:(issue#4397): container query variable names

[puckowski]

What:

• Fix for issue @container ruleset spaces removed after container-name #4397 container query with variable names like:

@varfoo: foo;
@threshold: 400px;
@container @varfoo (min-width: @threshold) {
    #sticky-child {
      font-size: 75%;
    }
}

Why:

Current behavior results in:

/* trailing space after the variable gets removed
    => 'foo' handled as a function
    => fail */
@container foo(min-width: 1024px){
    /* ... */
}

which is incorrect.

With the fix the output becomes:

/* space chars preserved */
@container foo (min-width: 1024px){
    /* ... */
}

Checklist:

• Documentation
• Added/updated unit tests
• Code complete