Unicode line breaking: Implement rules LB15a and LB15b

The new rules were added in Unicode 15.1 (TR #14, revision 51).

The rules read:

    LB15a: (sot | BK | CR | LF | NL | OP | QU | GL | SP | ZW)
           [\p{Pi}&QU] SP* ×
    LB15b: × [\p{Pf}&QU] (SP | GL | WJ | CL | QU | CP | EX
           | IS | SY | BK | CR | LF | NL | ZW | eot)

Add two new line breaking classes LineBreak_QU_Pi and _QU_Pf to
represent quotation characters with context that matches left
side of LB15a and right side of LB15b respectively. This way
it is still possible to use the line breaking classes table.

Also add a coment about the original source of the line
break table.

Task-number: QTBUG-121529
Change-Id: Ib35f400e39e76819cd1c3299691f7b040ea37178
Reviewed-by: Edward Welbourne <[email protected]>
Reviewed-by: Mårten Nordheim <[email protected]>

Author: eugmes