Support TextUnit for SymbolLayer.textVariableAnchorOffset

[sargunv]

1. it doesn't work at all, blocked by SymbolLayer.textVariableAnchorOffset can't be set to any value but nil() #143
2. once it works, it needs to support expressions for array elements, blocked by Design Proposal: Allow creating arrays with expressions maplibre-style-spec#950

[westnordost]

Should the property be disabled (read: commented out) until the upstream issue is resolved?

[sargunv]

Until the first issue is resolved, yeah that's reasonable

[westnordost]

Oh, I though the whole thing would be blocked by an upstream issue, not only step two.

[sargunv]

The property can't be set at all without the first issue. The property can't support TextUnit in SP without the second issue.

[sargunv]

Tracking PR in the JS style spec for the second issue: maplibre/maplibre-style-spec#951

I haven't set up a MLN dev env locally to work on that side yet