Add ability to autocomplete secret references inline

[dethancosta]

The user will now see a greyed-out suggestion with a secret reference once they type op://.

A few notes:

• references are stored in plain text, although the only vault name stored is the current one, I understand that there are security concerns; any suggestions for improving this are welcome
• retrieval of the references when VS Code starts up is slow; I'm not sure how to speed this up, as asynchronous code is already being used, and the list operation on item doesn't directly return references; admittedly I am not super experienced with Typescript, so there may be an obvious optimization that I'm not aware of. Again, suggestions are welcome.

Resolves #170