MemoryFileSystem: create the file when appending to a missing path

[Sreekant13]

Opening a path that doesn't exist yet in append mode behaves differently on MemoryFileSystem than on LocalFileSystem and the builtin open:

import fsspec
fsspec.filesystem("file").open("/tmp/new.txt", "ab")   # creates the file
fsspec.filesystem("memory").open("/new.txt", "ab")     # FileNotFoundError

open(path, "ab") creates the file if it's missing, and LocalFileSystem inherits that, so having the memory backend raise FileNotFoundError here is surprising. This makes _open create an empty file when the path isn't in the store yet for append modes (a/ab/a+b), then position at the end as before. Read modes (rb/r+b) are unchanged and still raise.

Added a regression test covering the three append modes.

[martindurant]

Should make sure to seek to the end of the file. In the test, it is already at the end, but this is not guaranteed.

[Sreekant13]

Good point @martindurant, thanks! Pushed a commit that seeks to the end explicitly after creating the file, so it matches the existing-file append path and no longer relies on the new file being empty. Also updated the regression test to assert the position is at the end.