Create file/directory with error if it already exists

[a-sully]

Migrated from WICG/file-system-access#107 (see #2)

This could be used by code that wants to create a unique temporary file/directory. The code would generate names by adding a sequence number to a fixed prefix ("temp001"), and stop when it's able to create a new file/directory (as opposed to opening an already existing one).