feat: alta-from-json loader for raw JSON Resume input

[smur89]

Problem

The whole point of JSON Resume is portability — one resume.json file used across many renderers. Today the template requires the caller to hand-translate the JSON into a Typst dict (different quoting, content vs string distinction, etc.). That friction makes the JSON Resume compatibility story aspirational.

Proposed solution

Add a public entrypoint alta-from-json(json-source, labels: (:), preferences: (:)) that:

1. Reads or accepts a JSON string (or accepts an already-parsed dict).
2. Validates the shape against the supported subset (panicking with helpful messages on unknown sections).
3. Coerces strings → content where the renderer expects content (e.g. highlights[], summary).
4. Calls alta() with the result.

Usage:

#import "@preview/altacv:0.x.x": alta-from-json

#alta-from-json(read("resume.json"))

Alternatives considered

• A separate companion script — kills the "single typst compile" workflow.
• Make alta() accept JSON directly — overloads the entrypoint; better to keep concerns separate.