feat(developer): LDML tool for users to run the LDML tests

[srl295]

Provide an end user feature for running the LDML regression tests.

            <keystroke key="s"/>
            <check result="abc\u0022...s" />
            <keystroke key="t"/>
            <check result="abc\u0022...st" />

At present this is in ldml.cpp / ldml_test_source.cpp

[srl295]

need to cross link this with the LDML features.

[mcdurdin]

We have a variety of regression testing formats across the Keyman repositories, including user-facing formats. I'd like to consolidate these before we tackle this.

[darcywong00]

Moving to A18S19, blocked by #5016