Strings should not be valid rvalues in regexp statements

Currently `if [foo] =~ "bar"` is valid logstash configuration syntax. This is insane. We should be more strict and only support regexps as rvalues in this case.

Not sure if this is technically a 'bug' at this point, but it's very confusing!

No file, test, or entry point is named. Start by locating the Logstash configuration parsing and validation for regexp statements, then identify how rvalues are checked; done means a quoted string is rejected in this context while a regexp remains valid, with tests covering both cases.

Written by the indexing model from the issue text.