Fix calc multiple custom props with fallbacks

[tornqvist]

This fixes an issue where a calc with multiple var in it would replace the trailing parentheses with the last fallback it encountered.

An example

/** source */
.text--calc {
  width: calc((100% - var(--xxx, 1px)) + var(--yyy, 10px));
}

/** before fix */
.text--calc {
  width: calc((100% - 1px) + 10px 10px;
  width: calc((100% - var(--xxx, 1px)) + var(--yyy, 10px));
}

/** after fix */
.text--calc {
  width: calc((100% - 1px) + 10px);
  width: calc((100% - var(--xxx, 1px)) + var(--yyy, 10px));
}

[jonathantneal]

Thank you, @tornqvist !