feat: return DSL string for DataDatatype.__repr__()

[NickCrews]

fixes #10391

I can add more tests or otherwise modify them if you think wise, but probably I think what we have here is sufficient.

One sanity check could be a roundtrip check?

for dsl in ["int8", "uint8" ,...]:
    dt = ibis.dtype(dsl)
    assert dsl == str(dt)
    assert dsl == repr(dt)

[cpcloud]

See my comment on #10391. I don't think we should do this.