Add state persistance across webview reloads

[jlevere]

Right now, to reduce memory overhead, we destroy state when the webview unloads (when the editor pane goes out of view).

While the extention is fast enough to rebuild the state on loads without much noticable impact, it would be nice to use the vscode builtin mechanisms of state serialization with getState and setState as is mentioned here Vscode Webview Serialization

[jlevere]

Updated to use retainContextWhenHidden. This could be updated by either using the getState setState serialization mechanisms as mentioned above, or implement a callback system to initiate reparsing. Since we are now using async background loading of parsed data, we need to ask for the data again when the webview comes back into context.
Or just accept it as it is