Implement flex-wrap: balance and flex-line-count (CSS Flexbox Level 2)

[nicoburns]

Summary

Implements the CSS Flexbox Level 2 balancing features, Servo-only and gated behind a new layout.flexbox.balance pref (default off):

• flex-wrap now parses the full Level 2 grammar nowrap | [ wrap | wrap-reverse ] || balance. The property switches from a generated keyword property to a custom specified::FlexWrap type (which is also the computed value) with two new variants:

  pub enum FlexWrap { Nowrap, Wrap, WrapReverse, Balance, WrapReverseBalance }

  wrap balance, balance wrap, and bare balance all compute to Balance; wrap-reverse balance computes to WrapReverseBalance; nowrap balance is rejected. Serialization round-trips to the canonical forms balance / wrap-reverse balance. The balance keyword only parses when layout.flexbox.balance is enabled (always false on Gecko builds).
• New flex-line-count longhand (positive <integer>, initial 1, animatable by computed value), engine = "servo" with servo_pref = "layout.flexbox.balance", per https://drafts.csswg.org/css-flexbox-2/#flex-line-count-property.

Gecko compatibility: since flex-wrap is no longer a keyword property, gecko.mako.rs gains manual set/copy/reset/clone_flex_wrap impls mapping to StyleFlexWrap (the balance variants, unreachable on Gecko, fall back to Wrap/WrapReverse). flex-line-count is skipped entirely for Gecko.

Verified with cargo build --features servo; consumed by DioxusLabs/blitz to plumb both properties through to Taffy's balancing implementation (DioxusLabs/taffy#1105).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/b7bd5b365d054b87a6053872323832b2
Requested by: @nicoburns

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