Sandbox: Add ability to run from local repro

[shilman]

Issue: N/A

What I did

Now if the user has generated a local repro in their /repros directory,
we can use that as the basis for a sandbox

How to test

# first generate a local repro
yarn generate-repros-next --template cra/default-js

# potentially edit it by hand etc. here
# ...

# then create a sandbox using that local repro
yarn sandbox --template cra/default-js --from-local-repro --no-publish --no-start-verdaccio

[yannbf]

LGTM!