Turbopack: cache variables in linker (#75440)
Maintain a cache of `VariableId -> JsValue` (per-module), so that we
don't reevaluate variables (mostly, unless it's a local variable inside
a function call)
```
canary 82026906a6:
15,3gb
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 501.72s user 108.30s system 760% cpu 1:20.26 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 502.01s user 102.27s system 774% cpu 1:18.04 total
mischnic/analyzer-link-cache 66d7e1f00c:
15,3gb
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 490.36s user 104.63s system 731% cpu 1:21.34 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 489.22s user 106.89s system 780% cpu 1:16.39 total
```
