MEGA65_FTP: outsource storage card free space display to an own command

[nobruinfo]

Test Environment (required)

• Tools Platform: Windows
• Tools Version: (m65 -h) Version: 20250415.20-develo-7a0467c~
• MEGA65 Platform: (DevKit, MEGA65, NexysA7, xemu) all of 'em
• ROM Release: independent of it
• Core Commit: independent of it

Describe the bug

LGB made a #define around some lines of code because using bigger SD cards some users had to wait up to many seconds for a display. This information was show after using the dir command in mega65_ftp. This tickets now intents to make the stats of the filesystem a separate command, it will most probably used less often than dir.

To Reproduce
Steps to reproduce the behavior:

1. Go to mega65_ftp
2. Type dir
3. See the contents of the directory followed by its total of space used.
4. Additional info on free space is missing since being removed.

Expected behavior

1. Type df as a suggested *nix command to get stats for the whole data partition of the currently selected storage.
2. See an output like follows:

%d Dir(s), %d File(s), %d out of %d MB free (%d out of %d Cluster(s) free)

Screenshots

N/A

Additional context

N/A