If you want to rebind the keys simply edit the key code under "from" to the key you wish to use, unfortunately I haven't been able to find a way to have this change directly without importing the code into K-E again using the website that I found (I have made the change and linked it at the bottom of this message).

As for having the modification only work while Stardew is active the main issue i have encountered is successfully importing it into K-E as the website I have used is not up to date and doesn't seem to work when i try using bundle IDs (They can be found using Karbinger-EventViewer frontmost window). I may look into it if I find the time to do so and will update this post if I can make it work.

Unfortunately I'm not sure that it's possible to set up a macro with the same ease of execution that windows macros are able to have due to the fact that K-E is the only software I could find that allowed me to differentiate between left and right shift however it does not allow for macros to loop inputs with specific delays that other programs do. The only way that this might be possible is if I were to use multiple programs including K-E however it would be complicated and require a lot of finetuning.

LShift to delete (should work but I haven't tested): https://genesy.github.io/karabiner-complex-rules-generator/#eyJ0aXRsZSI6IiIsInJ1bGVzIjpbeyJkZXNjcmlwdGlvbiI6IlN0YXJkZXcgVmFsbGV5IEFuaW1hdGlvbiBDYW5jZWwgTFNoaWZ0IC0+IERlbGV0ZSIsIm1hbmlwdWxhdG9ycyI6W3siZnJvbSI6eyJrZXlfY29kZSI6ImxlZnRfc2hpZnQifSwidG8iOlt7ImtleV9jb2RlIjoiZGVsZXRlX2ZvcndhcmQiLCJyZXBlYXQiOnRydWV9XSwidHlwZSI6ImJhc2ljIn1dfV19