Allow access to the parser during the read phase

[chrisjsewell]

The primary use case here is for within directives (also to a lesser extent roles).

Context:

To be fair to docutils, it never actually exposes directives as an extension point to users, this is done by sphinx (mainly in Sphinx.add_directive and Domain.directives)

The "problem" with this is that

1. Directives are currently a pretty leaky abstraction; exposing a lot of the "internals" of the docutils rST parser, particularly when it comes to nested parsing or parsing from other sources (like docstrings).
2. The docutils rst parser itself is not the greatest when it comes to things such as nested parsing, and also source mapped logging

This has started to lead to a proliferation of "hacks" into the rst parser, to try to overcom these shortcomings.
To name a few:

• nested_parse_with_titles https://github.com/sphinx-doc/sphinx/blob/v7.3.0/sphinx/util/nodes.py#L327
• parse_generated_content https://github.com/sphinx-doc/sphinx/blob/v7.3.0/sphinx/ext/autodoc/directive.py#L84
• switch_source_input https://github.com/sphinx-doc/sphinx/blob/v7.3.0/sphinx/util/docutils.py#L362
• Include.run https://github.com/sphinx-doc/sphinx/blob/v7.3.0/sphinx/directives/other.py#L384
• Only.run https://github.com/sphinx-doc/sphinx/blob/v7.3.0/sphinx/directives/other.py#L330
• sphinx.ext.collapse https://github.com/sphinx-doc/sphinx/pull/10532/files#r1581911730

These are exceptionally difficult to reconcile withinin parser like Myst, or even alternative rST parsers

Allowing access to the parser would allow the possibility for this sort of code to be centralised and abstracted better on the parser implementation

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Also somewhat related #12238 by @n-peugnet

[picnixz]

In principle, I'm fine but does the publisher.parser object have a lot of references (meaning, is there any chance that we have circular references?)

[picnixz]

This one might conflict an existing temp_data specified by the user somewhere else (unless temp_data is only meant for us). In particular, this requires a CHANGES entry.

I think we should improve the way we store temporary data and use namespaces to avoid conflicts.

[chrisjsewell]

I changed the key to _parser, which should minimise the risk of conflict, and exposed it as a property of the BuildEnvironment, similar to docname

Also added a CHANGES entry

[chrisjsewell]

I think we should improve the way we store temporary data and use namespaces to avoid conflicts.

well thats a topic for another day 😅