Allow newlines and tabs in the argument list of the theme() function

[philipp-spiess]

We noticed an issue that the theme() function wourld not properly parse in CSS if you split the argument list over multiple lines. This is fixed by treating \n and \t the same as space:

.custom-font {
  font-family: theme(
    fontFamily.unknown,
    Helvetica Neue,
    Helvetica,
    sans-serif
  );
}

Test plan

Added tests, but also tried it in the Vite example: