Simple Asciidoc lexer following Asciidoctor recommended practiques:
https://asciidoctor.org/docs/asciidoc-recommended-practices/
Tested with Asciidoctor README.adoc file:
https://github.com/asciidoctor/asciidoctor/blob/main/README.adoc
With the next result: 
PD: Sorry but GitHub doesn't allow to create PRs from a branch (ancebfer/geany 1.38.1) to a tag (geany/geany 1.38.0) as desired.
You can view, comment on, or merge this pull request online at:
https://github.com/geany/geany/pull/2986
-- Commit Summary --
* <a href="https://github.com/geany/geany/pull/2986/commits/9cb51e617b1b6355a7b360af7832103c34203388">Adds a simple Asciidoc lexer for Scintilla</a>
-- File Changes --
M data/filedefs/filetypes.asciidoc (50) M scintilla/Makefile.am (1) M scintilla/include/SciLexer.h (25) M scintilla/include/Scintilla.iface (27) A scintilla/lexers/LexAsciidoc.cxx (384) M scintilla/src/Catalogue.cxx (1) M src/highlighting.c (2) M src/highlightingmappings.h (33)
-- Patch Links --
https://github.com/geany/geany/pull/2986.patch https://github.com/geany/geany/pull/2986.diff