This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.
One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.
Apologies for the enormous commit, but I don't see a way to split this
up nicely. In the vast majority of cases it's a simple change. A few
extra places can use TRY instead of manual error checking though. :^)
Using a file(GLOB) to find all the test files in a directory is an easy
hack to get things started, but has some drawbacks. Namely, if you add
a test, it won't be found again without re-running CMake. `ninja` seems
to do this automatically, but it would be nice to one day stop seeing it
rechecking our globbed directories.
