fix: do not trust spoofable Symbol.toStringTag in type dispatch

Every mode (src/index.js, src/full.js, src/lite.js, src/json.js)
picked its clone strategy from Object.prototype.toString.call(x),
which reads a user-settable Symbol.toStringTag. A plain object
declaring a recognized tag (eg "Map") was routed into that type's
clone branch and crashed on methods it doesn't have; one with an
unrecognized tag fell through every branch and was returned by
reference instead of cloned.

Also check the object's real constructor, which Symbol.toStringTag
cannot affect, so a plain object (or one with no prototype) is still
deep-cloned as a plain object no matter what tag it claims.

Fixes #54

test/suites/tostringtag.js

@@ -0,0 +1,42 @@
+import { suite } from 'uvu';
+import * as assert from 'assert';
+
+export default function (klona) {
+	const Spoofed = suite('Symbol.toStringTag spoofing');
+
+	// A `Symbol.toStringTag` klona special-cases (eg "Map") used to misroute a
+	// plain object into that branch and crash.
+	// @see https://github.com/lukeed/klona/issues/54
+	Spoofed('recognized tag :: plain object is cloned, not misrouted', () => {
+		const input = { a: 1, b: { c: 2 }, [Symbol.toStringTag]: 'Map' };
+		const output = klona(input);
+
+		assert.notEqual(output, input, 'must return a new object, not the input');
+		assert.equal(output.a, 1);
+		assert.deepEqual(output.b, { c: 2 });
+
+		output.a = 999;
+		output.b.c = 888;
+		assert.equal(input.a, 1, 'mutating output.a must not affect input');
+		assert.equal(input.b.c, 2, 'mutating output.b must not affect input');
+	});
+
+	// An unrecognized `Symbol.toStringTag` used to fall through every branch
+	// and be returned by reference instead of cloned.
+	// @see https://github.com/lukeed/klona/issues/54
+	Spoofed('unrecognized tag :: plain object is cloned, not returned by reference', () => {
+		const input = { a: 1, b: { c: 2 }, [Symbol.toStringTag]: 'Foobar' };
+		const output = klona(input);
+
+		assert.notEqual(output, input, 'must return a new object, not the input');
+		assert.equal(output.a, 1);
+		assert.deepEqual(output.b, { c: 2 });
+
+		output.a = 999;
+		output.b.c = 888;
+		assert.equal(input.a, 1, 'mutating output.a must not affect input');
+		assert.equal(input.b.c, 2, 'mutating output.b must not affect input');
+	});
+
+	Spoofed.run();
+}