Arduino String class: features missing (feature request)

hi,
for Arduino C++ Strings some features are missing:
.readStringUntil(EOL) or perhaps a new .readln(): to read an entire textline all at once until end of line, discarding any EoL/CR
(e.g., text lines may have \n or \r\n at EoL, so readStringUntil('\n') or readStringUntil('\r') may fail or mess it up)

additionally also helpful:
.insert(substr, pos, len)
.delete(pos, len)
and especially
.TokenArg(token, delimiter1, delimiter2) // edit: optionally also just named .arg()
// returns the value of a token, e.g. for a String containing analog=1234;
`String myArg = myString.TokenArg("analog", "=", ";") ` would return 1234 as a String.

OTOH, IMO .compareTo() is ambiguous, it returns +/-/0 for after/before/ident but completely unclear if or what when neither and they are completely different. Better if it returned NAN/-1/0/+1 for notIn/smaller/ident/bigger.

PS,
just recalled that the WebServer lib provides a method `.arg()` which works somehow like the requested `String.TokenArg()` functionality:
```
int Analog1=0;
String myArg = webserver.arg("analog");
if (myArg != "") {
Analog1 = myArg.toInt();
}
```
That could be also used as a pattern for a String class TokenArg function.

Das Issue nennt Arduino C++ String-Methoden, enthält aber keine Dateien oder Tests. Beginne mit der Überprüfung der bestehenden String-Klasse und des compareTo/read-Verhaltens und kläre anschließend mit den Maintainer die angeforderte API und die Semantik von Zeilenumbrüchen. Als erledigt gilt die Aufgabe, wenn der Umfang und das Verhalten der vorgeschlagenen Änderungen vor der Implementierung abgestimmt sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.