Add LFN support

[beregond]

No description provided.

[thejpster]

Thank you for this pull request!

I have some comments.

• The nested array that was added uses 416 bytes of stack, on top of the heapless String of length 255. Passing the headless::String in would allow the caller to decide how much space they are prepared to use, plus UTF-8 is a more efficient encoding than an array of char and we can avoid the duplication.
• I didn't see where this checks the LFN hash and sequence number to check the LFN is valid.
• We probably also need to invalidate LFN entries when we delete a file.
• LFNs are equally valid on FAT16 and we should handle both.
• We're going to need some tests ;)

I like the approach of doing the dir listing first. That makes sense.

[beregond]

Yes, I will provide code for fat16 as well as all the tests - for now it is a little mess (as I'm running this against actual SD card to understand fully what I'm trying to implement), but will do 👍