Fix file system bugs

[tobil4sk]

• Fix File.append on macos
  Closes File.append is wrong in js target with hxnodejs on macOS haxe#12573
  According to the posix standard, when using pwrite (in our case, passing this.pos to writeSync), the O_APPEND flag is ignored.
  Linux doesn't comply with this, but Mac does, so we have to make the code compatible by passing null as the position which means write will be used instead of pwrite.
  See:
  https://pubs.opengroup.org/onlinepubs/9799919799/functions/pwrite.html

The pwrite() function shall be equivalent to write(), except that it writes into a given position and does not change the file offset (regardless of whether O_APPEND is set)

• Revert FileSystem.deleteDirectory should be recursive #85, because haxe specifies that deleteDirectory only works on empty directories. It also didn't throw if the folder didn't exist.
  See: https://api.haxe.org/sys/FileSystem.html#deleteDirectory

• Create file in File.update if it doesn't exist
  This is specified by haxe and other targets also do this.

[Simn]

I would prefer if we changed the deleteDirectory specification to support recursion on all targets. It's one of these annoying API specifications that gets in the way of the user's clear intent, and it forces custom implementations of a version that does support recursion in many places.

[tobil4sk]

This was discussed in HaxeFoundation/haxe#5585, but was forgotten about and closed.

it forces custom implementations of a version that does support recursion in many places.

From a quick search, I can see python and node seem to be the only ones that support this within their std libraries. On remaining targets it would have to be implemented manually which would requires properly handling symlinks etc. It would not be trivial to write properly with all the proper tests covering it.

I do see why it is useful to have it, but if we do support recursion across targets, it should be opt in because:

• All target languages do not delete recursively by default, so this is the expectation for most people
• It would be a breaking change, users might rely on the function throwing if it's non-empty, and if stuff goes wrong data will be lost because deleteDirectory does not put stuff in the recycling bin.

I think hxnodejs should still comply with the current spec and require an empty directory by default. Then we can add an optional recursive: Bool argument here, and re-open that issue to gradually start adding it to other targets?