add option to have strict file creation behavior

[natebrennand]

Workaround for #373

Adds an option to have MemMapFS fail when creating a file in a directory that DNE.

[CLAassistant]

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[Frassle]

I'd consider it a bug that it ever allowed this in the first place. A file create on OsFs would fail, it would be less surprising if MemFs also had that behavior. We've had bugs slip by testing due to this difference, but I'm not sure we would have noticed that we need to turn strict mode on for MemFs if it was an option like this.

[natebrennand]

I agree that it's a bug, but fixing it would be a breaking change.
Dunno that a v2 is anywhere on the horizon

[Frassle]

fixing it would be a breaking change.

Obligatory xkcd: https://xkcd.com/1172/