Introduce ANSI writer for formatted console output

The ANSI writer (-t ansi) outputs a document formatted with ANSI control
sequences for reading on the console.

Most Pandoc elements are supported and printed in a reasonable way, if
not always ideally. This version does no detection of terminal
capabilities nor does it fall back to different output styles for
less-capable terminals.

Some gory details:

- Title blocks are formatted with modest extravagance in --standalone
  mode.
- Strong, Emph, Underline, and Strikeout spans are all formatted
  accordingly using SGR codes (which will be silently ignored by
  terminals that don't support them).
- Headings have somewhat arbitrary styles applied to them that
  probably need immediate improvement.
- Blockquotes and all flavors of list look pretty good.
- Code spans are colored magenta-on-white, which on the author's
  terminal looks kind of like the pinkish treatment of code spans used
  by many stylesheets. This probably isn't a good final decision.
- Code blocks are formatted by Skylighting's formatANSI using standard
  writer options and included directly in the output. This has some
  issues; see code comments.
- Links are printed with OSC 8 to create hyperlinks and colored cyan.
  The author's terminal automatically adds a dotted-underline to OSC 8
  hyperlinks, but only colors them differently on command-mouseover.
  Setting an underlined style on links may be more broadly accessible.
  OSC 8 support is not checked for, so on terminals not supporting it or
  with support disabled, the link text will be colored but not do
  anything and the links will not be printed.
- Images are displayed as their alt text. Support for the Kitty and
  iTerm 2 inline image protocols is planned. Supporting other terminals
  by using Chafa (https://hpjansson.org/chafa/) to print sixels etc would
  be cool too but the author would have to do some FFI stuff and it would
  add a dependency to Pandoc.
- Tables are replaced with a useless placeholder. Table output using
  box-drawing characters is desired.
- Subscripts and Superscripts are just parenthesized when accurate Unicode
  representations aren't available. Because these span types could have
  all kinds of semantics, there's not an obvious thing to do with them.
- Simple math is translated to Pandoc inlines using existing
  functionality.  An ambitious person could look into emulating the
  console-mode math output of a computer algebra system, or rendering each
  display math element as an image with TeX or Typst and including it, or
  some other thing.

1 files changed, 21 insertions, 0 deletions

diff --git a/data/templates/default.ansi b/data/templates/default.ansi
new file mode 100644
index 000000000..9f6ca96de
--- /dev/null
+++ b/data/templates/default.ansi
@@ -0,0 +1,21 @@
+$if(titleblock)$
+$titleblock$
+
+$endif$
+$for(header-includes)$
+$header-includes$
+
+$endfor$
+$for(include-before)$
+$include-before$
+
+$endfor$
+$if(toc)$
+$table-of-contents$
+
+$endif$
+$body$
+$for(include-after)$
+
+$include-after$
+$endfor$