Support Human Readable Chapter Files (The Ones You See Everyday on YouTube)

Expected behavior of the wanted feature

Start supporting human readable chapter files and actively look them on the same directory with extension .chp or .chap.
Human readable chapter files are somewhat of an implicit Internet standard that everybody understands on the fly.

The format would be [[hh:]mm:]ss[.ms] Chapter Title like usual (no HTML or weird formatting).

00:05:00 Chapter 01
00:10:00 Chapter 02
00:15:00 Chapter 03
00:20:00 Chapter 04

Alternative behavior of the wanted feature

I was thinking about creating a shell script that converts timestamps into subtitle files that I can use as chapter files.
The problem with this approach is that I can't use subtitles for the same audio/video.

1
00:00:00.000 --> 00:05:00.000
Chapter 01

2
00:05:00.000 --> 00:10:00.000
Chapter 02

I also happen to know of scripts that let you take as input timestamps and insert chapters into the video.
I'm not particularly fond of this approach either since metadata can be quite messy to work with on a daily basis.
Same argument for metadata formats such as .ffmetadata using --chapters-file option in mpv.

Log file

mpv 0.32.0 Copyright © 2000-2020 mpv/MPlayer/mplayer2 projects
built on UNKNOWN
ffmpeg library versions:
libavutil 56.51.100
libavcodec 58.91.100
libavformat 58.45.100
libswscale 5.7.100
libavfilter 7.85.100
libswresample 3.7.100
ffmpeg version: 4.3.2-0+deb11u1ubuntu1

The issue names no repository files, tests, or entry points. Begin by locating where mpv discovers sidecar files and parses chapter data, then trace how parsed chapters reach playback. Done means .chp and .chap files in the media directory are recognized and the stated timestamp-and-title format produces chapters.