LaTeX writer: support beamer overlays

[lawcho]

Support beamer overlays

This PR allows scripting beamer animations in markdown (and other formats).

The input syntax uses the on attribute, e.g.

# What type? {.t}

`parseInt :: ???`{.haskell on=1}

. . .

`parseInt :: String -> Int`{.haskell on=2-3}

. . .

`parseInt "haha" == ???`{.haskell on=3}

![](https://openclipart.org/image/800px/190136){on=3}

. . .

`parseInt :: String -> Maybe Int`{.haskell}

`parseInt "haha" == Nothing`{.haskell}

This compiles to the following beamer-LaTeX:

\begin{frame}[fragile,t]{What type?}
\phantomsection\label{what-type}
\only<1>{\VERB|\OtherTok{parseInt ::} \OperatorTok{???}|}

\pause

\only<2-3>{\VERB|\OtherTok{parseInt ::} \DataTypeTok{String} \OtherTok{{-}\textgreater{}} \DataTypeTok{Int}|}

\pause

\only<3>{\VERB|\NormalTok{parseInt }\StringTok{"haha"} \OperatorTok{==} \OperatorTok{???}|}

\includegraphics<3>{https://openclipart.org/image/800px/190136}

\pause

\VERB|\OtherTok{parseInt ::} \DataTypeTok{String} \OtherTok{{-}\textgreater{}} \DataTypeTok{Maybe} \DataTypeTok{Int}|

\VERB|\NormalTok{parseInt }\StringTok{"haha"} \OperatorTok{==} \DataTypeTok{Nothing}|
\end{frame}

Related issues

• Add capability to bypass beamer frame in section slides not just for "notes" slides #8118 (similar feature proposed, but div-specific)
• Beamer overlays in notes #7848 (similar feature proposed, but note-specific)
• Beamer overlays (\only, \uncover, etc) #3184 (similar feature proposed, discussion of work-around using raw LaTeX)

Coverage

Tested:

• Block images
• Inline code

TODO:

• Test tables
• Test divs
• Test spans
• Test links
• Test inline images
• Debug fenced code blocks (currently broken)
• Finalize syntax (currently on=OVERLAY attribute, but open for bike-shedding)
• Write regression tests

Feedback welcome.

[lawcho]

Closing in favor of #9215, which is simpler and more robust.