[css-text] Phrase line breaking should fallback to normal line breaking if overflow

We've got a feedback that when a phrase can't fit in a line, it should fallback to normal line breaking.

For example:

<div style="width: 5em">こんにちは。</em>

This line overflows when the phrase engine computes "こんにちは" as a phrase.

Authors can prevent the overflow by:

div {
  word-break: phrase;  /* the keyword TBD */
  overflow-wrap: anywhere;
}

but this produces:

こんにちは
。

which breaks the Japanese line breaking rules not to break before the fullwidth period. From authors' point of view, this should break as below instead:

こんにち
は。

Since the exact algorithm to determine the phrase boundaries is UA-dependent, this could be either part of the spec or part of the UA's algorithm, whichever works for me but wanted to share the feedback.

@litherum @frivoal @fantasai