[css-overflow-4] line-clamp `max-lines` - support `auto` value to fit container

[mozilla-apprentice]

A resolution was made for csswg-drafts/#9368.

**[css-overflow-4] line-clamp `max-lines` - support `auto` value to fit container **

• RESOLVED: Change the line-clamp shorthand syntax to allow omitting the max-lines value

Discussion.

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[dholbert]

No action needed; we don't support max-lines yet, per:
https://searchfox.org/mozilla-central/rev/2fa4750e87d49ec7ad19fa1da167a03ef6d5e954/servo/components/style/values/generics/box.rs#128-132

/// Note that we only implement -webkit-line-clamp as a single, longhand
/// property for now, but the spec defines line-clamp as a shorthand for
/// separate max-lines, block-ellipsis, and continue properties.
///
/// https://drafts.csswg.org/css-overflow-3/#line-clamp