LaTeX: Allow linebreaks at \ (in literals, code-blocks, parsed-literal)

Default configuration is to allow the linebreak after the backslash.
It is possible at user side via 'preamble' key and some LaTeX code
to allow the linebreak before the backslash, but each of inline-literal,
code-block, and parsed-literal has its more or less cumbersome way for
that.

Closes: #6000, #6001

This commit handles only the \ character, which had been left aside
by accident so far. Inline-literals could break at more characters
but this is not yet done in this commit. In this context, in future
it could be useful to modify sphinx.util.texescape to use mark-up
such as \sphinxtextbackslash rather than naked \textbackslash and
similarly for some other characters. This could then be used to
allow linebreaks even in regular text paragraphs.

doc/latex.rst

@@ -619,12 +619,15 @@ macros may be significant.
     default ``true``. Allows linebreaks inside inline literals: but extra
     potential break-points (additionally to those allowed by LaTeX at spaces
     or for hyphenation) are currently inserted only after the characters
-    ``. , ; ? ! /``. Due to TeX internals, white space in the line will be
-    stretched (or shrunk) in order to accomodate the linebreak.
+    ``. , ; ? ! /`` and ``\``. Due to TeX internals, white space in the line
+    will be stretched (or shrunk) in order to accomodate the linebreak.
 
     .. versionadded:: 1.5
        set this option value to ``false`` to recover former behaviour.
 
+    .. versionchanged:: 2.3.0
+       added potential breakpoint at ``\`` characters.
+
 ``verbatimvisiblespace``
     default ``\textcolor{red}{\textvisiblespace}``. When a long code line is
     split, the last space character from the source code line right before the