Add netCDF extension nc

[axelboc]

As requested on the HDF Group forum: https://forum.hdfgroup.org/t/vscode-extension-for-hdf5/9769/8

[loichuder]

Given the plethora of HDF5 extensions out there, this is bound to happen again.

Would there be a way to have something similar than for the h5wasm demo ? Extensions encouraged but not enforced ?

If not, we should add something in the README to indicate which extensions are supported (and why) and how a simple renaming can bypass this.

[axelboc]

Yep... Unfortunately, the custom editor contribution point only allows providing an array of glob patterns: https://code.visualstudio.com/api/references/contribution-points#contributes.customEditors

Good point on adding the list to the readme.

[axelboc]

I didn't explicitly suggest renaming files, as I think explaining that it relies on filename extensions is clear enough.