Migrates the alignment conversions to taffy's new safe-alignment style
types: `AlignItems`/`AlignContent` and friends are now structs pairing a
keyword with a safety modifier, constructed via associated constants
(e.g. `AlignItems::START`) instead of enum variants.

The motivating change in this release is [taffy
#911](https://github.com/DioxusLabs/taffy/commit/df2663aad35801f7522f6878a0300646dfe624f7)
("More correct caching logic"), which keys taffy's per-node measure
cache on axis, parent size, and available space. Today GPUI is largely
immune to the old cache-key conflation because text answers every sizing
probe with the same width; the follow-up PR in this stack (honest
min-content text measurement) returns different sizes for different
constraints, which makes cache-key correctness a prerequisite. 0.12.1
additionally hotfixes two block layout/caching regressions in 0.12.0,
and 0.11.0's grid fix (resolving item percentages against the grid area
rather than the container) comes along as well.

Stack:
1. **This PR** — taffy 0.12.1
2. #60722 — honest min-content text measurement
3. #60723 — CSS `auto` grid tracks for table-like column sizing

Release Notes:

- N/A