Add command to show the current configuration

[sdavids]

Similar to:

$ git config -l

---

The proposed command would show the actual configuration being used.

With

.swift-format

{
  "version": 1
}

$ swift-format show-configuration

would display

{
  "version": 1,
... default values ...
}

---

$ swift-format dump-configuration

displays the default configuration.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://126948332