gh-102130: Support a tab completion for Libedit in cmd, site, and pdb.

[Constantin1489]

After I dug the rabbit hole, I wanted to share something and commit it to cpython. I have my research for the .python_history compatibility problem. I will write comments within days.

My commit allows a tab complete in pdb and cmd for Libedit readline and improves compatibility between GNU readline and Libedit.

For better compatibility, Python Documents should suggest using backslashed escape sequences(e.g. \t, \a) or the ASCII character corresponding to the octal number(or octal excape sequences).(e.g. \011, It's because the only thing GNU readline(man bash and /Readline Key Bindings to search the part.) and Libedit(man editrc) have in common.

tab of readline.parse_and_bind("tab: complete") is a symbolic character name. Libedit doesn't support a number of symbolic character names(like TAB, ESC, DEL).

Also, unlike emacs keybinding, GNU readline doesn't support control characters of the form '^character' (e.g. '^I' it's a tab).
Both GNU readline and Libedit support the following backslashed escape sequences.

\a     bell
\b     backspace
\f     form feed
\n     newline
\r     carriage return
\t     horizontal tab
\v     vertical tab
\nnn   octal escape sequences

GNU readline's predefined commands are in the '/Readline Command Names' of man bash.
Libedit's predefined commands are in the section "EDITOR COMMANDS" of man editrc. But there are other undocumented commands in its source code.
Or you can get a list by import readline; readline.parse_and_bind('bind -l') (Libedit only).

• Issue: cmd.py command completion does not work with libedit #102130

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📚 Documentation preview 📚: https://cpython-previews--107738.org.readthedocs.build/

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