[rust] Remove short flags in selenium-manager

[bonigarcia]

Description

This PR removes the use of short flags (e.g., -b for browsers) leaving only the long flags (e.g., --browser).

Motivation and Context

There is a quite long lists of CLI flags at the moment in selenium-manager (e.g. --browser, --driver, etc.), and this list is supposed to be longer in the future. For this reason, it is difficult to find meaningful short flavour of these options. Moreover, there are two options (--driver-ttl and --browser-ttl, both created by external PRs) without the short flag.

This PR unifies the criteria for the selenium-manager flags, removing all the short flags. Also, this PR updated the rust README (which contains the --help of selenium-manager).

NOTE: I checked that all bindings use the long flags (i.e., --browser or --driver), so this change should not break anything in the bindings.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[diemol]

Thank you, @bonigarcia!