Fix issues flagged by lint rules

[ecraig12345]

In the beachball next (v3) branch, I enabled eslint. This PR brings back some of the fixes made in response to lint rules:

• use optional catch bindings (remove (err) from catch (err) if not used)
• replace non-null assertions with optional chaining or other checks
• fix shadowed names
• prefer const
• ensure the correct types are used instead of implicit or explicit any
• remove unused imports and parameters
• use explicit module boundary types

(I didn't enable eslint in master/v2 because in next I'm using eslint v9 flat config, which is probably not compatible with Node 14, and I'm not sure it's worthwhile to back-port the config.)