Refine API + nomenclature

[ds300]

Initial thoughts

• new Atom(name, val, opts?) => atom(name, val, opts?)
• new Derivation(name, derive, opts?) => computed(name, val, opts)
• @derivation => @computed(opts?)
• Add @atom(opts?)
• Replace useDeepComparisons with isEqual option, to avoid depending on lodash.isEqual (also prevents bundle bloat for folks using lodash already)
• (unsure about this one) replace .get() and .set(val) with .value and .value = val, or just replace .get() and leave .set() alone.
• diffSinceEpoch(epoch) => getDiffSince(epoch)
• remove Atom#update, barely useful, a smaller api is more valuable.
• remove buildIncrementalDerivation

[ds300]

We should avoid exposing the classes Atom and Derivation and create interfaces for the public API parts to avoid folks relying on implementation details.

[ds300]

tldraw/tldraw-lite#964