Move connections pane into core positron

[dfalbel]

Addresses #4874

This PR is an initial implementation of the connections pane into Positron core. It's still missing a lot of functionality, but will eventually be equivalent to the current positron-connections extension.

TODO:

• [x] Workspace storage of previous connections
• [x] Setting to opt-in for the new implementation
• [x] Button to open data explorer for a Table or View
• [ ] Codicons for most common DB data types (schema, collections, etc) and for creating a new connection
• [x] Review event listeners disposing
• [x] Implement Python side of new GetMetadata RPC
• [x] Implement refresh connections
• [x] Display the field type
• [x] Display the language for each connection
• [x] Review error handling
• [x] Fix smoke tests (smoke tests are passing locally)
• [x] Fix integration tests
• [x] Rename 'Connections Core' to just 'Connections'
• [x] Support for the focus event