Feature Request: Add startup G-code to IS Calibration Tool

The [Input Shaping Calibration Pattern](https://marlinfw.org/tools/input_shaping/freq-calibr.html) seems to work well once you know what to look for. There are three issues I've found: 1. Not knowing what/how to interpret the results. I believe there's work ongoing on this. 2. Recommended ranges for the test. This could/should maybe be addressed in the item above. My Ender 3v2 printed ringing towers best at frequencies X:59.75, Y: 32.1. The Y frequency was indicated easily on this test, the X was not since it was close to the default max frequency. Maybe I could have avoided printing 10+ ringing towers... 3. Startup G-code included is very basic and (in my case) doesn't work well. Could we get a text box to include our own G-code? Right now I'm editing the files by hand, and if it was only one file one time it would be fine, but it isn't. Every time I change a value I have to go and remove the generated startup G-code and add my working G-code. Many slicers and thus their startup G-code run in relative extrusion mode, this generated calibration code runs in absolute extrusion. Could we get a M82 at the start of the generated code (aka, after any startup G-code) to make sure the extruder is using the right set of numbers? This is not a bug/issue, more of a belt-n-suspenders solution.