GeezerVison

…roductionlist` (pythonGH-127835)

As a first step toward aligning the grammar documentation with Python's actual
grammar, this overrides the ReST `productionlist` directive to:
- use `:` instead of the `::=` symbol
- add syntax highlighting for strings (using a Pygments highlighting class)

All links and link targets should be preserved. (Unfortunately, this reaches
into some Sphinx internals; I don't see a better way to do exactly what
Sphinx does.)

This also adds a new directive, `grammar-snippet`, which formats the snippet
almost exactly like what's in the source, modulo syntax highlighting and
keeping the backtick character to mark links to other rules.
This will allow formatting the snippets as in the grammar file
(file:///home/encukou/dev/cpython/Doc/build/html/reference/grammar.html).

The new directive is applied to two simple rules in toplevel_components.rst

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(cherry picked from commit 58a4357)

Co-authored-by: Petr Viktorin <[email protected]>
Co-authored-by: Blaise Pabon <[email protected]>
Co-authored-by: William Ferreira <[email protected]>
Co-authored-by: bswck <[email protected]>
Co-authored-by: Adam Turner <[email protected]>