Support supplying a watchFactory option that resolves the plugin and uses it for watching file or directory

[sheetalkamat]

With this change we allow users to use custom watchFactory plugin either on command line or in editor
For plugin writers the plugin is expected to be module that returns the factory with methods to watchFile and watchDirectory

export type UserWatchFactoryModule = (mod: { typescript: typeof ts, options: WatchOptions, config: any }) => UserWatchFactory;
export interface UserWatchFactory {
    watchFile?(fileName: string, callback: FileWatcherCallback, pollingInterval: number, options: WatchOptions | undefined): FileWatcher;
    watchDirectory?(fileName: string, callback: DirectoryWatcherCallback, recursive: boolean, options: WatchOptions | undefined): FileWatcher;
    onConfigurationChanged?(config: any): void;
}

This behaves similar to language service plugin as in user can have this set in tsconfig.json in watchOptions as watchFactory, pass it on command line during batch compilation or set in vscode's global watchOptions

watchFactory option can either be module name to look for or it can be PluginImport which is object literal with property name that is resolved as name and the object literal is passed on to the factory.

The sample watchFactory using @parcel/watcher is at https://github.com/sheetalkamat/typescript-parcel-watcher

The changes in PR include:

• Adding option watchFactory in watchOptions that takes in either string or object literal. The name of the factory needs to be package name to resolve (just like our LS plugins`
• During watchFile or watchDirectory we look for this resolved module and use the specific function if available.
• On command line we look for the module next to typescript we are running from
• In editor it looks for paths just like it would find LS plugins from (eg. depending on options, custom locations, executing typescript peer module or next to tsconfig.json)
• In editor we also support calling onConfigurationChanged similar to we call for LS plugins when editor uses protocol to do so.

Eg plugin using parcel watcher is at https://github.com/sheetalkamat/typescript-parcel-watcher

Installing:

git clone https://github.com/sheetalkamat/typescript-parcel-watcher.git
cd typescript-parcel-watcher
npm i
npm run build

Usage:

npm link
cd repoWithTypeScriptCode
npm link typescript-parcel-watcher
# install typescript from this PR

At this point you can pass --watchFactory typescript-parcel-watcher to tsc or set it in watchOptions of config file and/or vscode settings.

[typescript-bot]

Thanks for the PR! It looks like you've changed the TSServer protocol in some way. Please ensure that any changes here don't break consumers of the current TSServer API. For some extra review, we'll ping @sheetalkamat, @amcasey, @mjbvz, @minestarks for you. Feel free to loop in other consumers/maintainers if necessary

[sheetalkamat]

Close in favor of #54012