feat(core): add optional token

[Fyzu]

Sometimes some dependencies are optional, but now there is no way to specify such dependencies correctly.

My suggestion is to add an optional token:

const foo = tokenOptional('foo')<'foo'>()

const bar = injectable(foo, (foo: 'foo' | undefined) =>
  foo !== undefined ? `${foo}bar` : 'baz'
)

bar({}) // ✅ valid

[Fyzu]

@raveclassic what do u think?

[raveclassic]

@Fyzu Hm, such case should be already supported by the regular token function: const foo = token('foo')<'foo' | undefined>(), we just need to correctly produce UnknownDependencyTree with optional: true.