This is a stripped-down template for 1.18 Fabric+Forge mods. It's heavily derived (aka, taken wholesale) from HexCasting's excellent project layout.
This (intentionally) has very few dependencies beyond those necessary for its config loader and for Kotlin compatibility, and almost no actual functionality, but does have a lot of frameworks that can be easily expanded, and can be compared to the existing HexCasting code to see actual solutions for real-world situations. This also means that there are a number of pieces of relevant code commented out: there's a simple config system bundled in here (and one of the few unremoved dependencies), which you should only use if you actually need it.
The approach here does not actually avoid the various complexities of cross-modloader programming. You still need to very carefully maintain functions that are distinct between the two environments. It's just a very well-designed approach for separating these fields, without having to duplicate a lot of work.
There will be some follow-up work to turn it from a generic "Hello World" mod into your Generic "Hello World" mod. Some keywords to look at:
yourmod.name.here is the default package name. While valid for test purposes, you will need to change this before release (and ideally immediately after forking) or risk (near-certain) collisions and annoying bugs, and it's easier the earlier you change it.
IntelliJ (and most other IDEAs) will allow bulk refactoring, though expect to need to change a few package settings at the top of files.
See this page for information about package naming rules, particularly don't use a popular TLD that you don't actually own or control.
yourmod is the default modID. AFTER you've refactored your package name, this is the next thing to change. It's only referenced in a few places, but other mods, compile-time behavior (both Forge and Fabric) and maven will interact with it heavily.
See this page for some naming conventions specific to this ID.
After you've made these changes, it's safe to leave YourAPI and YourConfig as their current names, but do feel free to rename them; something specific to your modId or your package will make regularly importing them less obnoxious.
Your Mod is the default mod name. This is more for aesthetics than code relevance, but it does show up in a good few places and it's kinda embarrassing if it's left at default.
Check your mods.toml (for Forge) and fabric.mod.json (for Fabric) regarding other mod-specific settings, such as author, icon, dependencies you want presented to the user, so on. There's a lot I'm not going to summarize here.
Actually Running:
Once you've made those updates, you'll need to reload Gradle. Do it a couple of times, for safety's sake.
The automatically generated Run configurations won't work. Sorry. You'll need to use Gradle->Fabric>Tasks->Fabric->run* or Gradle->Forge->Tasks->forgegradle runs->run*. There's probably a way to fix this, but good luck.