Fix getters being imitated as undefined, and td.replace on getter properties

[rosston]

Fixes #526

The problem

Any property defined with a getter came out of imitation as undefined. The property copier read only descriptor.value, and accessor descriptors have get instead. That affected every entry point that imitates: td.replace(path) (including testdouble-jest's td.mock), td.object, td.constructor and td.imitate.

The most visible case is compiled CommonJS:

• SWC compiles every export to a non-configurable getter. Projects built with SWC (Next.js, @swc/jest) couldn't stub their own modules at all: td.when(dep.doTheThing()) threw dep.doTheThing is not a function.
• Babel and tsc compile re-exports (export { x } from, export *) to getters, so barrel files broke the same way.
• Native ESM through td.replaceEsm was unaffected, because namespace objects use data properties.

A related bug: td.replace(object, 'prop') replaced and restored by assignment. That throws Cannot set property ... which has only a getter when prop is a getter with no setter.

What changed

Imitating getters (copy-props.js, overwrite-children/index.js)

The original object is now passed down to copyProps, which calls each getter on it and imitates the result. Functions become test doubles and objects are imitated recursively, as for data properties. It falls back to undefined, which was the old behavior, when:

• the getter, or imitating its result, throws
• the accessor has only a setter
• the object being walked is a prototype. There's no instance to call the getter on, so td.constructor(Foo) instances still read prototype getters as undefined
• getters are nested more than 3 deep. A getter can return a fresh object with getters of its own forever (get kid () { return new A() }), so the depth is capped to keep imitation finite

Replacing getter properties (replace/property.js)

For a getter with no setter, td.replace(object, 'prop') now defines the fake instead of assigning it:

Getter	Replace	td.reset()
Own, configurable (tsc re-exports)	Redefined as a data property	Original descriptor restored exactly
Inherited (class getter, via an instance)	Shadowed by an own property	Own property deleted
Own, non-configurable (SWC exports)	Clear error pointing to td.replace('path/to/module')	n/a

Data properties and getters with setters are still replaced by assignment, as before.

Behavior changes

• Getters now run during imitation, so their side effects run too. For example, Node deprecation warnings from getters on fs or crypto, or lazy loading. Getters that throw still give undefined.
• td.replace(moduleObject, 'export') can't work on SWC output, because JavaScript won't let a non-configurable getter be redefined. It now fails with an error explaining that and suggesting td.replace(path), which does work for these modules after this change.

Review notes

• The depth cap uses a module-level counter. That's safe because imitation is fully synchronous, and a finally restores the counter even when something throws. Its one quirk: a getter that calls td.imitate itself starts that inner imitation partway up the cap, so it gets fewer levels of expansion.
• A 2017 commit (adea88c) switched to descriptor.value specifically to avoid running getters. The hazards it named are handled elsewhere now:
  • gatherProps stops at Function.prototype and Object.prototype.
  • Current V8 makes arguments/caller data properties.
  • The try/catch covers anything that still throws.