[partition]: restore defaultFileSystemType in config

Add the old key back, document the new key, expand schema.

src/modules/partition/partition.conf

@@ -137,6 +137,14 @@ initialSwapChoice: none
 # If nothing is specified, Calamares defaults to "ext4".
 #
 # Names are case-sensitive and defined by KPMCore.
+defaultFileSystemType:  "ext4"
+
+# Selectable filesystem type, used when "erase" is done.
+#
+# When erasing the disk, the *defaultFileSystemType* is used (see
+# above), but it is also possible to give users a choice:
+# list suitable filesystems here. A drop-down is provided
+# to pick which is the filesystems will be used.
 availableFileSystemTypes:  ["ext4", "btrfs", "f2fs"]
 
 # Show/hide LUKS related functionality in automated partitioning modes.

src/modules/partition/partition.schema.yaml

@@ -18,6 +18,8 @@ properties:
     alwaysShowPartitionLabels: { type: boolean, default: true }
 
     defaultFileSystemType: { type: string }
+    availableFileSystemTypes: { type: array, items: { type: string } }
+
     enableLuksAutomatedPartitioning: { type: boolean, default: false }
     allowManualPartitioning: { type: boolean, default: true }
     partitionLayout: { type: array }  # TODO: specify items