Add a vscode setting for "disable autorebuild"

For people that like to run bsb outside of rls.
There's already a setting in rls `autoRebuild`, but there's no way to set it through vscode at the moment.

- in `NotificationHandlers.re`, `"workspace/didChangeConfiguration"`, parse out an `autoRebuild` parameter and add it to the [settings record](https://github.com/jaredly/reason-language-server/blob/master/src/lsp/NotificationHandlers.re#L122).
- in the vscode plugin's [package.json](https://github.com/jaredly/reason-language-server/blob/master/editor-extensions/vscode/package.json#L45) add a `reason_language_server.autoRebuild` setting, defaulted to `true`.

cc @rusty-key

Start in src/lsp/NotificationHandlers.re at the workspace/didChangeConfiguration handler and inspect the settings record, then update editor-extensions/vscode/package.json. The change is done when autoRebuild can be configured through the VS Code setting and defaults to true.

Written by the indexing model from the issue text.