elixir allows metavariables as keywords

[amchiclet]

Previously, keywords (as in keyword/value pairs) didn't allow metavariables, so cases like

  foo($KEYWORD: $VALUE)
  %{$KEYWORD: $VALUE}

would fail to parse.

This PR adds support for that.

Also made ellipsis its own rule, as previously suggested at #466.

Tested with make test and added a test.

Security

• Change has no security implications (otherwise, ping the security team)

[amchiclet]

The workflow build-on-commit failed because some CircleCI thing that we need to fix.

But I ran the steps needed for that workflow locally and all tests passed.

[amchiclet]

The CircleCI is still failing (we haven't moved to GHA yet), so I ran the steps needed for that workflow locally and all tests passed.

Merging now.