feat(demo/ufmt): add some usefull missing methods: (Fprintx, AppendX, SprintX, PrintX) (#4015)

This PR enhances the `ufmt` package by adding missing methods such as
`FprintX`, `AppendX`, `SprintX`, and `PrintX`. These additions address
the package's previous limitations. It also centralizes and improves the
logic in a printer, similar to the approach used by the `fmt` package.

Additionally, I have added new tests and moved the logic of `Println`
test into `Sprint` test (since the logic has been moved into the
printer, we can now test it using the `Sprint` function)

Note: I added these functions mainly because I enjoy using `fmt.FprintX`
with `strings.Builder` and `bytes.Buffer`.

Author: gfanton