Create a UI for Presto connections

[whscullin]

Currently the only way to configure a Presto connection is via environment variables.

The existing connection editors live at:
https://github.com/malloydata/malloy-vscode-extension/tree/main/src/extension/webviews/connections_page
and the VS Code configuration lives https://github.com/malloydata/malloy-vscode-extension/blob/main/package.json#L391