HTML API: Simplify the class_list loop

[sirreal]

This is a simplification and optimization of the class_list loop based on some work in #7857.

The important change here is that the position ($at) advances past any whitespace characters before the while condition is checked. The loop never begins at whitespace characters allowing for a few checks to be eliminated and the overall loop logic to be simplificed. Whitespace is skipped before the loop is entered and when $at is adjusted at the end of the loop.

This allows for some conditionals to be removed from the loop because they become invariants. The loop is always exited by the while condition after the entire string has been processed.

Trac ticket: https://core.trac.wordpress.org/ticket/65846

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