slightly improve code thanks to Go 1.18 APIs

strings.Cut makes some string handling code more intuitive.
Note that we can't use it everywhere, as some places need LastIndexByte.

Start using x/exp/slices, too, which is our first use of generics.
Note that its API is experimental and may still change,
but since we are not a library, we can control its version updates.

I also noticed that we were using TrimSpace for importcfg files.
It's actually unnecessary if we swap strings.SplitAfter for Split,
as the only whitespace present was the trailing newline.

While here, I noticed an unused copy of printfWithoutPackage.

position.go

@@ -10,8 +10,9 @@ import (
 	"go/parser"
 	"go/printer"
 	"path/filepath"
-	"sort"
 	"strings"
+
+	"golang.org/x/exp/slices"
 )
 
 func isDirective(text string) bool {
@@ -120,8 +121,8 @@ func printFile(file1 *ast.File) ([]byte, error) {
 	})
 
 	// We add comments in order.
-	sort.Slice(toAdd, func(i, j int) bool {
-		return toAdd[i].offset < toAdd[j].offset
+	slices.SortFunc(toAdd, func(a, b commentToAdd) bool {
+		return a.offset < b.offset
 	})
 
 	copied := 0