Bugs in using escape characters

micro:bit-support: 21346
Describe the bug
In blocks entering within Serial Write Line, Serial Write String on in a general string assignment, entering a character escape sequence such as “\r” adds extra “\”
If the character required is \”, then two extra \ are added.

Entering a character escape sequence in Javascript gives an unrendered block sometimes.
A round trip from JS to blocks back to JS results in a null being added at the front i.e if the string is “\r” then the round trip results in “” + “\r”

Entering hex number as a character escape crashes program and all work is lost e.g. for control-Z “\0x1a”

There is a documented block https://makecode.microbit.org/reference/math/from-char-code to enter a character code that renders if you use eg let ctrlZ = String.fromCharCode(26) and switched to bklocks, but this doesn't appear in the Text Block menu. Can this be added?

Reproduce the reported cases in Serial Write Line, Serial Write String, general string assignments, and the JavaScript-to-blocks round trip. Check the existing math/from-char-code entry point and the Text Block menu. Done means escape sequences round-trip correctly, hexadecimal input does not crash or lose work, and the documented character-code block is available in the Text Block menu.