Add '=' as valid label value separator symbol

[rin67630]

Currently the only valid label value separator symbol for Serial Plotter is ':'

https://github.com/arduino/Arduino/blob/ba34eb640e5f6c2f8618debf61022f731c0eab74/build/shared/ArduinoSerialPlotterProtocol.md

Beside the fact that ':' may conflict with time formats, the '=' is a natural label postfix.
"Value 1=123.45"

Adding this label value separator symbol would make many reports work ooB in Serial Plotter.

Thank you for considering