👌 Allow access to the parser during the read phase

[chrisjsewell]

This PR allows access to the Parser instance during parsing, using the read-only property BuildEnvironment.parser.

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 overcome 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 with other parsers 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]

I'm fine with this approach ! (though we'd still need to think about the temp_data management in general...)