zcbor_decode.c: Add a function for printing CBOR data

[oyvindronningstad]

zcbor_diag_print() prints the CBOR data pointed to by the state variable, marked up with color and indentation.

Repurpose zcbor_any_skip() for the traversal. Add a new file zcbor_diag_print.h to hold new helper functions.

Add test for printing function.

Add info about the function to the README.

To see the README additions rendered: https://github.com/oyvindronningstad/zcbor/tree/diag#diagnostic-printing-of-cbor-data

[oyvindronningstad]

Now depends on #373

[oyvindronningstad]

Exploring other solutions, closing for now.