Scala: allow ellipsis in for loop headers

[mmcqd]

See semgrep/semgrep#5661

Security

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

[linear]

PA-1579 Scala for loops parse error

I expected that for($EXPR2; $IDENT <- $GEN; $EXPR) {yield ... }
Or for(...; $IDENT <- $GEN; ...) {yield ... }

would parse as patterns but they don't as of 0.100: https://semgrep.dev/s/NoNx
From semgrep/semgrep#5650
Originally opened by ievans

[aryx]

I would have added a (* semgrep-ext: *) comment, to indicate this is actually not part of Scala but
an AST extension useful for semgrep only.