bug#73724: [PATCH] Add folding support for begin and end macros

[Paul Nelson]

This patch supports the folding of \begin{...} and \end{...} macros.

The naive way to attempt such folding in AUCTeX via string
specifications in TeX-fold-macro-spec-list has many drawbacks.  I
would be happy to elaborate upon these, but do not do so in this
message.

This patch adds function specifications to TeX-fold-macro-spec-list
and a new user-facing option TeX-fold-begin-end-spec-list that
controls how to fold begin/end arguments.  To get a flavor, see the
screenshots at https://github.com/ultronozm/czm-tex-fold.el (but let
me note that I've streamlined the code while preparing this patch).

I'll illustrate here with some examples:

- If the option TeX-fold-begin-end-spec-list is nil, then behavior is as before.

- If the option contains an item such as

    (("↴" . "↲") ("itemize" "enumerate" "description" "frame"))

  then, e.g., \begin{enumerate} and \end{enumerate} fold to ↴ and ↲,
respectively.

- If the option contains an item such as

    ((TeX-fold-format-theorem-environment . "◼")
         ("note" ("theorem" "thm")))

  then

  - \begin{note} folds to "Note",
  - \begin{theorem} and \begin{thm} fold to "Theorem",
  - \begin{theorem}[Foo] and \begin{thm}[Foo] fold to "Theorem (Foo)".
  - \end{note}, etc., fold to "◼".

I've supplied what I believe to be reasonable defaults (attempting to
err on the conservative side), but would welcome any feedback.

Thanks, best,

Paul

0001-Add-folding-support-for-begin-and-end-macros.patch
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