[Feature] Add command to upgrade to the latest swc binary

[maelanleborgne]

Having a fixed version made sense 2 years back when the swc binaries weren't always available for all platforms depending on the release, which seems to have been fixed since then.
Having a command to upgrade to the latest version (if not fixed in the config) may be interesting for the user and save some time (checking out the latest release version on github, setting it in the config, removing the existing binary and launching the build command).