Plugins - notify a plugin when a tsconfig.json file changes

[egamma]

A plugin can define plugin specific properties in the plugin section of the tsconfig.json file. When the user edits a tsconfig.json then the plugin should be notified so that it can react to setting changes.

[mhegazy]

Piping events from tsserver to the plugin regarding changes to the config file that in sections that do not touch project configuration requires some reworking of the server. Today we make multiple assumptions about what changes require a requery for errors vs others. It also seems that the plugins in general need to monitor other files, e.g. tslint.json for the tslint plugin. I would recommend adding a file watcher on the tsconfig.json as well, and triggering a refresh of errors if the lint configuration section changes. With #15914 in (PR: #23824), the plugin should be able to trigger a refresh to errors using project.refreshDiagnostics.

[egamma]

@mhegazy having the plugin watch the tsonfig.json makes sense. Is there a way for a plugin to find out which tsconfig.json file should be watched?

[mhegazy]

Calling getProjectName on a ConfiguredProject should give you the path to the tsconfig.json/jsconfig.json file path. The path is also stored on CompilerOptions.configFilePath ( that is an internal property, so does not show in the .d.ts, but we can make it visible through a helper).