Added lfs_fs_grow for growing the filesystem to a different block_count

The initial implementation for this was provided by kaetemi, originally
as a mount flag. However, it has been modified here to be self-contained
in an explicit runtime function that can be called after mount.

The reasons for an explicit function:

1. lfs_mount stays a strictly readonly operation, and avoids pulling in
   all of the write machinery.

2. filesystem-wide operations such as lfs_fs_grow can be a bit risky,
   and irreversable. The action of growing the filesystem should be very
   intentional.

---

One concern with this change is that this will be the first function
that changes metadata in the superblock. This might break tools that
expect the first valid superblock entry to contain the most recent
metadata.

lfs.h

@@ -732,6 +732,16 @@ lfs_ssize_t lfs_fs_size(lfs_t *lfs);
 // Returns a negative error code on failure.
 int lfs_fs_traverse(lfs_t *lfs, int (*cb)(void*, lfs_block_t), void *data);
 
+#ifndef LFS_READONLY
+// Grows the filesystem to a new size, updating the superblock with the new
+// block count.
+//
+// Note: This is irreversible.
+//
+// Returns a negative error code on failure.
+int lfs_fs_grow(lfs_t *lfs, lfs_size_t block_count);
+#endif
+
 #ifndef LFS_READONLY
 #ifdef LFS_MIGRATE
 // Attempts to migrate a previous version of littlefs