Fix errors, add fatfs modified time

[daniel-j]

• remove _ftello_r implementation as it collides when linking. This fixed building for me.
• rename define PATH_MAX since it collides with another define.
• Add support for reading file modified timestamp using stat (fatfs only).

LittleFS could also support file timestamps through attributes although this would have to be implemented as an extra feature.

I'm not using pico-vfs from C directly but I made a Nim wrapper for it that I'm working on in my Pico SDK wrapper
https://github.com/daniel-j/picostdlib/blob/master/src/picostdlib/pico/filesystem.nim
https://github.com/daniel-j/picostdlib/tree/master/examples/filesystem

_ftello_r linking error:

/usr/lib/gcc/arm-none-eabi/14.1.0/../../../../arm-none-eabi/bin/ld: /usr/lib/gcc/arm-none-eabi/14.1.0/../../../../arm-none-eabi/lib/thumb/v6-m/nofp/libg.a(libc_a-ftello.o): in function `_ftello_r':
/build/arm-none-eabi-newlib/src/build-newlib/arm-none-eabi/thumb/v6-m/nofp/newlib/../../../../../../newlib-4.4.0.20231231/newlib/libc/stdio/ftello.c:88: multiple definition of `_ftello_r'; CMakeFiles/test_pico.dir/picostdlib/src/picostdlib/vendor/pico-vfs/src/filesystem/vfs.c.obj (symbol from plugin):(.text+0x0): first defined here

Nice work on pico-vfs!