lib/filterx: add startswith / endswith functions

[OverOrion]

@version: 4.8

log {
  source {stdin(flags(no-parse));};
  filterx {
    declare needle = "bar";
    declare path = "";

    if (startswith($MSG, "foo", ignorecase=true)) {
      path = "starts with foo";
    }
    elif (startswith($MSG, needle)) {
      path = "starts with bar";
    }
    elif (endswith($MSG, "foo", ignorecase=true)) {
      path = "ends with foo";
    }
    elif (endswith($MSG, needle, ignorecase=true)) {
      path = "ends with bar";
    }
    else {path = "NOPE";};
  };
  filterx {vars();};
  destination { file("/dev/stdout"); };
};