Uighur hyphenation should be supported

[r12a]

This issue is applicable to Uighur.

Unlike Arabic, which is never hyphenated, words in Uighur text in the Arabic script can be broken at line ends. A short horizontal stroke is added at the end of the line, separated from the previous text by a small space, and joining forms are retained for left-joining letters at line end and line start.

This hyphenation method needs to be supported in browsers.

Specs:
css-level-3 provides controls for hyphenation, and alludes to the requirement to create joining letter forms at line end and start for Arabic-script text where hyphenation is allowed, but leaves it to the browser implementation to produce the specific type of hyphenation that is appropriate to a given language.

Tests & results:
The following tests use the second half of the text in the image shown above.

interactive test, hyphens:auto makes the browser hyphenate Uighur text and uses a low stroke at the line end, and joining forms at line end and start.

Results:

• Gecko: ❌ No hyphenation occurs Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:98.0) Gecko/20100101 Firefox/98.0
• Blink: ❌ No hyphenation occurs Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.74 Safari/537.36
• Webkit: ❌ No hyphenation occurs Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.3 Safari/605.1.15

interactive test, After setting hyphens:manual, the browser hyphenates Uighur text where soft hyphens occur. Hyphenation is shown by a low stroke at the line end, slightly separated from the foregoing text, and joining forms at line end and start.

Results:

• Gecko: ✅❌ The lines break and the line-end and line-start letters have joining forms, but the marker used is an ordinary hyphen and not on the baseline. Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:98.0) Gecko/20100101 Firefox/98.0
• Blink: ✅❌ The lines break and the line-end and line-start letters have joining forms, but the marker used is an ordinary hyphen and not on the baseline. Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.74 Safari/537.36
• Webkit: ✅❌ The lines break but the line-end and line-start letters don't have joining forms, and the marker used is an ordinary hyphen and not on the baseline. Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.3 Safari/605.1.15

Browser bug reports:
Gecko • Blink • Webkit

Priority:
Uighur hyphenation is common in printed material, so it should also work on the Web.

[r12a]

The first comment in this issue contains text that will automatically appear in one or more gap-analysis documents as a subsection with the same title as this issue. Any edits made to that comment will be immediately available in the document. Proposals for changes or discussion of the content can be made in comments below this point.

Relevant gap analysis documents include:
Uighur