Translate message in pyexec and use invisible characters to convey serial state to tools

[tannewt]

Follow up from #5307. CP 7 broke Thonny and other serial based tools because it relies on certain messages. Translating these messages breaks the tools. Instead, we should add invisible characters that these tools can used to determine serial link state (human repl, raw repl vs paste mode). These characters will remove the dependence of these tools on the messages that may change from version to version.

cc @aivarannamaa