fix(7986): nested css-calc and css-variables with fallback give incorrect result - #7987

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What is the current behavior?

Nesting css-calc expressions with css-variable with fallback give the wrong result.

Assuming input value: calc(calc(var(--undefined-var, 16) / 2) * 2).

1. _evaluateCssVariableExpression returns calc(calc(16) / 2) * 2) (note: the missing end-))
2. _evaluateCssCalcExpression returns 8 * 2).
3. Then the value is cast to number with parseFloat which results in the number 8.

The problem is caused by cssVariableExpressionRegexp finding the wrong ).

What is the new behavior?

Fixes #7986.

1. No longer uses regexp to process css-variable expressions.
2. Switch to simple string operations e.g. includes, indexOf and lastIndexOf.
3. Process css-variable expressions is now done from the end of the string, before it was done from the start.
   3b. This means the inner css-variable is a nested expression is processed first, but this was the simplest way to find the correct end-) of those expressions.