`Object.assign` over functions breaks

[simon-abbott]

When using better-typescript-lib v2.11.0, passing a function to Object.assign breaks TS callability inference.

This code works fine in base TS, and runs perfectly, but it breaks the build with better-typescript-lib:

const foo = () => 1;
const bar = Object.assign(foo, { baz: 1 });
bar();

[graphemecluster]

Currently the Writable type accidentally excludes function signatures from the sources (which is correct), but is unfortunately also affecting the target. And yes, it might be worthwhile to retain the mutability of the properties in the target, so I think the fix to this issue is to change the return type to T & Intersect<Ts>.