Even though I'm not retired yet, I'm "old enough to remember the ISPF editor on IBM mainframes": I still use (and love) it more or less every day ...
To actually answer your question, I suggest you have a look at SPFLite. Here is a quote about it, from Wikipedia:
Editors used in the PC environment typically operate in a significantly different ...
CudaText editor (free). It has almost all the needed items. Except
it has its own UI look
it cannot find symbols in other JS files. But it can do it for the current file.
It has also the helper for JS coding - plugin Tern, for some intellisense.
The CudaText (free) and Sublime Text both can be used so, but with plugin. Plugin is not existing yet.
The tip how to write this plugin for CudaText: plugin must handle on_key event (better for only keycodes of LEFT/RIGHT, it's option in install.inf). In this event, plugin must disable default action by returning False, and run its own action by using ed.cmd(...
You might want to try
display simultaneously file tree and editor area (and be able to switch between files); CHECK
support markdown format (highlight markdown format and display formatted preview); CHECK
(nice to have) be able to display TOC generated from markdown headers in file; CHECK
be open source and work under Linux. CHECK
CudaText (uvviewsoft.com , open source, cross platform) is rather light weight.
If you remove python DLL from the Windows version, it will start even faster. But all plugins will not work then. It’s look is not heavy, it is like in Notepad3.
Latex is capable of doing this, if you are willing to go that route. An example:
Il contratto di trasporto concluso ai sensi delle Condizioni Generali,
di norma, è relativo all’utilizzo da parte del passeggero di un singolo
treno ed è rappresentato da un titolo di viaggio che legittima ...
CudaText editor allows this, using two addons:
lexer for subtitles. In the menu "Plugins / Addon Manager / Install" you will find lexer for SRT subtitles.
plugin to auto-complete words from the current file. In the same menu: Complete From Text.
Not limited to subtitles, PhraseExpress is a text expander program for multiple platforms, including Windows and MacOS. There is a limited free version, although it's not particularly clear what features are not part of the free program.
My sister is a transcriptionist and uses this software to expedite her recording from audio. It works in a simple manner. ...
Try micro, a versatile text editor with no dependencies, (IOW a fully static binary).
Easy to Use
Micro's number one feature is being easy to install (it's just a static binary with no dependencies) and easy to use.
Use a simple json format to configure your options and rebind keys to your liking. ...
CudaText editor (free, cross platform) allows this. Using plugin "Insert Pics".
Install plugin from menu: Plugins / Addon Manager / Install, enter "insert pics".
Call plugin commands from menu: Plugins / Insert Pics.
Readme is at https://github.com/CudaText-addons/cuda_insert_pics/tree/master/readme
Example screenshot of text with pictures:
Would this be hard to achieve in Google Apps Script (given that I currently know nothing about Apps Scripts)?
If something like this is hard or not to achieve is "primarily opinion" considering only that you tried to do this using AppScript (I know nothing about it) and that you know nothing about Apps Script, but...
This could be done in Google Apps ...
ReText is a simple editor for Markdown and reStructuredText markup languages. ReText is open source and cross platform, and it is in the default repositories of many popular Linux distributions. ReText supports tabs, live text preview and syntax highlighting. Supported additional export formats are HTML, ODT and PDF.
I used ReText for years. It is ...