feat: add PackageUpgrader interface for targeted package upgrades

Add optional PackageUpgrader interface with UpgradePackages method
that upgrades specific installed packages (unlike Upgrade which
upgrades everything).

Each backend uses native upgrade commands:
- apt: apt-get install --only-upgrade
- dnf: dnf upgrade <pkg>
- pacman: pacman -S <pkg>
- apk: apk upgrade <pkg>
- pkg (FreeBSD): pkg upgrade <pkg>
- flatpak: flatpak update <pkg>
- snap: snap refresh <pkg>

Non-installed packages are filtered out and returned as Unchanged.

Closes #31

pacman/pacman_other.go

@@ -49,3 +49,7 @@ func isInstalled(_ context.Context, _ string) (bool, error) {
 func version(_ context.Context, _ string) (string, error) {
 	return "", snack.ErrUnsupportedPlatform
 }
+
+func upgradePackages(_ context.Context, _ []snack.Target, _ ...snack.Option) (snack.InstallResult, error) {
+	return snack.InstallResult{}, snack.ErrUnsupportedPlatform
+}