LibJS: Fix PropertyName::from_value() for negative and non-int numbers

It was converting *any* number to an i32 index, which obviously is not
correct for negative ints, doubles, infinity and nan.

Fixes #3712.

Libraries/LibJS/Tests/object-expression-computed-property.js

@@ -0,0 +1,12 @@
+test("Issue #3712, negative/non-int computed property in object expression", () => {
+    const o = {
+        [1.23]: "foo",
+        [-1]: "foo",
+        [NaN]: "foo",
+        [Infinity]: "foo",
+    };
+    expect(o[1.23]).toBe("foo");
+    expect(o[-1]).toBe("foo");
+    expect(o[NaN]).toBe("foo");
+    expect(o[Infinity]).toBe("foo");
+});