fix: escape backticks in package comment for generated code

[Yanhu007]

Fixes #537

Problem

When a magefile's package comment contains backticks, the generated mage_output_file.go breaks:

// Source magefile:
// This is a magefile, to be called with `go tool mage`.
package main

// Generated code (broken):
_fmt.Println(`This is a magefile, to be called with `go tool mage`.
`)

The backticks in the comment terminate the raw string literal, producing invalid Go.

Fix

Use %q (Go quoted string) instead of embedding in a raw backtick literal:

// Before:
_fmt.Println(`{{.}}\n`)

// After:
_fmt.Println({{printf "%q" .}} + "\n")

This properly escapes any special characters including backticks.

Testing

All mage tests pass.