Improve --mapdir ergonomics / Add support for --mount

[fschutt]

It's never clear which way --mapdir and [fs] sections should map: guest:host or host:guest? It would be better to enforce it either via typing or via very clearly named structs.

[ptitSeb]

--mapdir was here for compatibility reason (with wasmtimes).
Having a syntax like Docker or linux mount would be much simpler and unambigious.

[ptitSeb]

--mapdir will need to be still supported but marked as deprecated

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Feel free to reopen the issue if it has been closed by mistake.