Add support for GDAL vsimem

[mmomtchev]

This PR adds support for GDAL vsimem - a in-memory filesystem It allows a JS Buffer object to be opened with gdal.open as if it were a file The Buffer is automatically registered in the virtual memory file system and it is automatically unregistered on garbage collection In order to keep persistence, every in-memory dataset keeps a reference to its buffer in gdal.Dataset.buffer

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[yocontra]

Looks great - I don't see any issues with the code. Outstanding items before merge:

• Can you add docs for this?
• Could you add a few test cases for bad input? (Empty buffer, invalid buffer contents, etc.)

[yocontra]

@mmomtchev Tests are failing on some of the CI

[yocontra]

Published as 2.6.0 - thanks again!