Add HTML5 and Linux to CI, improvements to development env

[EliteMasterEric]

Dependant on #2205.

This pull request provides the following improvements:

• Github Actions for continuous integration now build properly, resolving Improve Github continuous integration #2207.
• Added Github Actions for continuous integration of HTML5 and Linux. These now both build properly as well.
• Added VSCode run configurations for Windows, Linux, and HTML5, with debug and release modes.
• The code now works on Linux. In order to do this, I had to debug and reposition several compile-time conditional blocks, which was frustrating because I had no way of knowing which defines were for what purpose, thus the below change.
• Reworked the use of defines to be more human-readable. Instead of using #if cpp or #if desktop, a single readable define is created in project.xml for each use case, such as FEATURE_DISCORD, which defines the circumstances where it is active. Then, that define is used in code (for example, every instance of #if desktop surrounding the use of a DiscordClient call now uses #if FEATURE_DISCORD). This makes it easier for developers and maintainers to ensure code is properly included and excluded from certain builds in an intuitive manner.

Please get back to me if you have any concerns on this.

[EliteMasterEric]

Marked as draft until #2205 is approved and merged.

[Kade-github]

thank you