Bugs in using escape characters

[microbit-mark]

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?

[nhzandi]

Considering escape characters, in the Blockly API it is handled like this:

string = block.getFieldValue('TEXT');
string = string.replace(/\\/g, '\\\\')
                 .replace(/\n/g, '\\\n')
                 .replace(/'/g, '\\\'');

So Blockly is intentionally adding an extra \ to escape the characters, maybe assuming that children using it are not familiar with the concept, and since it makes problem when communicating with a serial port and they don't have this problem. A quick fix to this would be to handle it in this function:

function compileText(e: Environment, b: Blockly.Block, comments: string[]): JsNode {
     return H.mkStringLiteral(b.getFieldValue("TEXT")); /* <-- HERE*/
}

A fix for this could be sth like this:

b.getFieldValue("TEXT").replace('\\r', '\r')  /* for other characters as well*/

or could have special characters as separate blocks and use join_text to join them together.

So if everyone agrees with that.

[microbit-mark]

^^ @jwunderl @abchatra