This is promarily being done to unblock #9223.

Prior to this, we'd validate that the user's `startingDirectory` existed
here. If it was invalid, we'd gracefully fall back to `%USERPROFILE%`.

However, that could cause hangs when combined with WSL. When the WSL
filesystem is slow to respond, we'll end up waiting indefinitely for
their filesystem driver to respond. This can result in the whole terminal
becoming unresponsive.

Similarly, with #9223 we want users to be able to specify WSL paths in a
profile, but this bit of validation logic totally prevents that from working,
because it'll just replace the path with `%USERPROFILE%`.

If the path is eventually invalid, we'll display warning in the
`ConptyConnection`, when the process fails to launch.

Closes #9541
Closes #9114

…t-validate-paths