Intent to prototype and ship: CSS overscroll-behavior value, "chain"

[Kanaru Sato]

Summary:
Add a value, "chain", to overscroll-behavior property. This is a value which
disables local boundary default actions, while allowing non-local ones. In
short, it can be said as the inversion of "contain".

Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=2036966

Specification:
https://drafts.csswg.org/css-overscroll-behavior/#propdef-overscroll-behavior

Standards Body:
CSS WG

Platform Coverage:
All

Preference:
None

DevTools Bug:
N/A

Extensions Bug:
N/A

Use Counter:
None

Standards-Positions Discussion:
https://github.com/mozilla/standards-positions/issues/1406

Other Browsers:
    Blink: Shipped
    WebKit: They haven't clarified their positions yet
            (https://github.com/WebKit/standards-positions/issues/661)

web-platform-tests:
- https://wpt.fyi/results/css/css-overscroll-behavior/parsing
- https://wpt.fyi/results/css/css-overscroll-behavior/overscroll-behavior.html
- https://wpt.fyi/results/css/css-overscroll-behavior/overscroll-behavior-without-overflow.html