Use go:generate for generating cheatsheets

This has several benefits:
- it's less code
- we're using the same mechanism to generate all our auto-generated files, so if
  someone wants to add a new one, it's clear which pattern to follow
- we can re-generate all generated files with a single command
  ("go generate ./...", or "make generate")
- we only need a single check on CI to check that all files are up to date (see
  previous commit)

pkg/cheatsheet/generator.go

@@ -0,0 +1,14 @@
+//go:build ignore
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/jesseduffield/lazygit/pkg/cheatsheet"
+)
+
+func main() {
+	fmt.Printf("Generating cheatsheets in %s...\n", cheatsheet.GetKeybindingsDir())
+	cheatsheet.Generate()
+}