Dex/loadData: Merge objects w/ spread syntax

[larry-the-table-guy]

Results in fewer object shapes.

Similar to #10608. Smaller effect, but very easy change.

loadData and construct all are in milliseconds.
heap 1 is MB after loading data for all mods (after)
heap 2 is MB after constructing objects for all mods (aftest)

Stage	loadData	construct all	heap 1	heap 2
Baseline	1261	3421	174	277
just loadData (this PR)	1265	3052	171	268
just field order (other PR)	1274	2197	174	263
both PRs	1264	2064	171	260

---

Benchmark code

Inserted at bottom of ./sim/dex.ts

const v8 = require('v8');
v8.writeHeapSnapshot(); // before
const start1 = performance.now();
Dex.includeMods();
Dex.includeModData();
const loaded_data_t = performance.now();
v8.writeHeapSnapshot(); // after
const start2 = performance.now();
for (const d in dexes) {
        if (d === 'base') continue;
        const m = dexes[d];
        m.species.all();
        m.moves.all();
        m.items.all();
        m.abilities.all();
        m.types.all();
        m.natures.all();
}
const constructed_t = performance.now();
v8.writeHeapSnapshot(); // aftest

console.log('loadDatas', loaded_data_t - start1);
console.log('constructors', constructed_t - start2);
throw new Error('stop early');

[Zarel]

Thanks!