Add migrate command (#207)

* Add base and loader command

* Support updating to latest

* Support explicit fabric versions

* Support explicit Forge version

* Support quilt

* Add support for updating Minecraft

* Add support for Forge recommended

* Fix Forge version sorting

* Various fixes

* Add documentation to the use

* More suggestions

migrate/minecraft.go

@@ -0,0 +1,70 @@
+package migrate
+
+import (
+	"fmt"
+	packCmd "github.com/packwiz/packwiz/cmd"
+	"github.com/packwiz/packwiz/cmdshared"
+	"github.com/packwiz/packwiz/core"
+	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
+	"os"
+)
+
+var minecraftCommand = &cobra.Command{
+	Use:     "minecraft [version]",
+	Short:   "Migrate your Minecraft version to a newer version.",
+	Aliases: []string{"mc"},
+	Args:    cobra.ExactArgs(1),
+	Run: func(cmd *cobra.Command, args []string) {
+		modpack, err := core.LoadPack()
+		if err != nil {
+			// Check if it's a no such file or directory error
+			if os.IsNotExist(err) {
+				fmt.Println("No pack.toml file found, run 'packwiz init' to create one!")
+				os.Exit(1)
+			}
+			fmt.Printf("Error loading pack: %s\n", err)
+			os.Exit(1)
+		}
+		currentVersion, err := modpack.GetMCVersion()
+		if err != nil {
+			fmt.Printf("Error getting Minecraft version from pack: %s\n", err)
+			os.Exit(1)
+		}
+		wantedMCVersion := args[0]
+		if wantedMCVersion == currentVersion {
+			fmt.Printf("Minecraft version is already %s!\n", wantedMCVersion)
+			os.Exit(0)
+		}
+		mcVersions, err := cmdshared.GetValidMCVersions()
+		if err != nil {
+			fmt.Printf("Error getting Minecraft versions: %s\n", err)
+			os.Exit(1)
+		}
+		mcVersions.CheckValid(wantedMCVersion)
+		// Set the version in the pack
+		modpack.Versions["minecraft"] = wantedMCVersion
+		// Write the pack to disk
+		err = modpack.Write()
+		if err != nil {
+			fmt.Printf("Error writing pack.toml: %s\n", err)
+			os.Exit(1)
+		}
+		fmt.Printf("Successfully updated Minecraft version to %s\n", wantedMCVersion)
+		// Prompt the user if they want to update the loader too while they're at it.
+		if cmdshared.PromptYesNo("Would you like to update your loader version to the latest version for this Minecraft version? [Y/n] ") {
+			// We'll run the loader command to update to latest
+			loaderCommand.Run(loaderCommand, []string{"latest"})
+		}
+		// Prompt the user to update their mods too.
+		if cmdshared.PromptYesNo("Would you like to update your mods to the latest versions for this Minecraft version? [Y/n] ") {
+			// Run the update command
+			viper.Set("update.all", true)
+			packCmd.UpdateCmd.Run(packCmd.UpdateCmd, []string{})
+		}
+	},
+}
+
+func init() {
+	migrateCmd.AddCommand(minecraftCommand)
+}