Use Object.assign() for JSX spread

[developit]

Edge doesn't support Object spread properties. The JSX plugin here generates them, which means we're producing code that breaks in Edge.

Most users have @babel/proposal-object-rest-spread configured, and would see JSX spread transpiled to Babel's full object spread output with helpers. If they've configured that plugin with useBuiltIns:true, the result is clean Object.assign calls, though this is inconsistent with the spec behavior of spread properties (but not JSX Spread).

Instead, this PR changes preset-modules to transpile JSX Spread Attributes into Object.assign() calls, which are natively supported in all ES Modules-supporting browsers. The old behavior is still available, but is now gated behind a new useSpread option in the plugin (not the preset).

before:

// input:
const X = <div {...a} b />

// preset-modules output:
const X = <div {...{ ...a, b: true }} />;

// final output (after JSX transform):
function ownKeys(object, enumerableOnly) { /* snip */ }
function _objectSpread(target) { /* snip */ }
function _defineProperty(obj, key, value) { /* snip */ }
const X = h("div", _objectSpread({}, a, { b: true }));

// final output (after *loose* JSX transform):
var _assign = Object.assign || function(){ /* snip */ }
const X = h("div", _assign({}, a, { b: true }));

Size (uncompressed): 1.1kB

after:

// input:
const X = <div {...a} b />

// preset-modules output:
const X = <div {...Object.assign({}, a, { b: true })} />;

// final output (after JSX transform):
const X = h("div", Object.assign({}, a, { b: true }));

Size (uncompressed): 80 bytes

---

h/t @devongovett who noticed this!