LaTeX writer: support beamer overlays (take 2)

[lawcho]

This PR is a successor to #9203, with a different implementation.

This PR allows ergonomic beamer scripting, via attributes on pandoc elements.

• The key-value attributes [only,visible,uncover,invisible] are interpreted specially by the beamer writer
• These can be applied to any (attribute-carrying) Block or Inline element
• The generated beamer-LaTeX code for those elements is wrapped in \begin{XXXenv}<YYY>...\end{XXXenv},
  for each (special) attribute pair (XXX,YYY)

Example

I can write a presentation in markdown:

---
header-includes: |
  \setbeamercovered{transparent=25}
---


# How to print a tree {.t}

`{-# LANGUAGE StandaloneDeriving #-}`{.haskell visible=5-}

`data Tree a = Leaf a | Branch [Tree a]`{.haskell}

![](https://upload.wikimedia.org/wikipedia/commons/1/1c/Haskell-Logo.svg){only=1}

[*Many* options:]{invisible=1-5}

```{.haskell visible=3,6-}
  deriving (Show)
```

```{.haskell visible=4,6-}
instance (Show a) => Show (Tree a) where
  show :: Tree a -> String
  show (Leaf x) = "Leaf (" ++ show x ++ ")"
  show (Branch ts) = "Branch " ++ show ts
```

`deriving instance (Show a) => Show (Tree a)`{.haskell visible=5-}

## Discussion {visible=7-}

::::: {uncover=8-}
* Not all at once
  * `OverlappingInstances`
  * [Choose max. 1!]{only=9}
:::::
::::: {uncover=11-}
* See also
  * [Tweag on run-time instances](https://www.tweag.io/blog/2021-04-08-capabilities-ad-hoc-interpreters/){only=12-}
:::::

Then pandoc src.md -o slides.pdf --to beamer generates a 12-page slideshow:

slides.pdf

[jgm]

I guess invisibleenv, etc., are built-in beamer environments, requiring no special definition?

[lawcho]

Yes, invisibleenv, onlyenv, etc. are built-in beamer environments.

N.B. after writing this PR, I managed to get similar results with a lua filter:

-- Convert ELEM{only=X} to \begin{onlyenv}<X>ELEM\end{onlyenv}
-- (and likewise for several other overlay types)

local overlays_supported = pandoc.List
  {"only","uncover","visible","invisible","action","alert"}

local function tex_wrap(block_mode,pre,el,post)
  return block_mode
      and pandoc.Blocks
    {pandoc.RawBlock("tex",pre), el, pandoc.RawBlock("tex",post)}
      or pandoc.Inlines
    {pandoc.RawInline("tex",pre), el, pandoc.RawInline("tex",post)}
end

local function wrap_in_overlays(block_mode,el)
  for k,v in pairs(el.attributes) do
    el = overlays_supported:includes(k)
      and tex_wrap(block_mode,"\\begin{"..k.."env}<"..v..">",el,"\\end{"..k.."env}")
      or el
  end
  return el
end

local function wrap_block(el) return wrap_in_overlays(true,el) end
local function wrap_inline(el) return wrap_in_overlays(false,el) end

return {{
  -- These block elements have attributes
  CodeBlock = wrap_block,
  Div = wrap_block, 
  Figure = wrap_block,
  Table = wrap_block,
  -- These inline elements have attributes
  Code = wrap_inline,
  Image = wrap_inline,
  Link = wrap_inline,
  Span = wrap_inline,
}}

So I'm no longer sure that this PR is worth merging.

[lawcho]

8 presentations later (half of these), a lua filter still covers my animation requirements.

So I'm closing this PR as not-wanted.

[jgm]

If you wouldn't mind keeping this open, it's still something I'd consider...it could be a useful feature in pandoc.