[ upstream commit 869e678b1ae3461b169259155e3bb52b6b4fa072 ]

Add a "Copy Commands" to some code blocks. This new button attempts to
copy only commands (and not their output) to the clipboard. The
distinction between commands and output relies on the presence of a
prompt symbol, either "$" or "#", at the beginning of the commands. If a
command ends with a trailing backslash, copy the next line as well.

For example, the following snippet:

    .. code-block:: shell-session

        $ ls -l
        foo
        cat
        $ echo 1 \
        2 \
                3\
            4
        $nospace
        # exit

should place the following text into the clipboard:

    ls -l
    echo 1
    2
            3
        4
    exit

The button is added for the following blocks, when they contain several
lines and at least one command is found in the block:

- "code-block", but with language "shell-session" only,
- Literal blocks ("::"),
- Parsed literals.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Sebastian Wicki <sebastian@isovalent.com>