runtime: Update files for ConTeXt, METAFONT, and MetaPost.

runtime: Update files for ConTeXt, METAFONT, and MetaPost.

This update is meant to be included in the upcoming 9.2 release.

**New**

- Support ConTeXt's convention to optionally specify an output directory
  in a comment line at the beginning of a source file.
- If a log file is not found, Vim does not create a new buffer.
- Removed `syntax/shared` files for the following reasons:
  - they are not necessary for the plugin to work (they only improve
    over existing syntax highlighting);
  - they are relative large;
  - they can be automatically (re)generated by users at any time using
    ConTeXt (explained in the doc);
  - since ConTeXt is updated frequently, they quickly become obsolete.

**Minor**

- Prefer `var` to `const` inside functions.
- Prefer `$`-interpolation to `printf()`.
- All revision dates set to the same date for consistency.
- Updated the error format.
- Various tweaks to the documentation, but nothing disruptive or new.

closes: #19148

Signed-off-by: Lifepillar <lifepillar@lifepillar.me>
Signed-off-by: Christian Brabandt <cb@256bit.org>