Plugin localization service does not await on init

[kittaakos]

What it does

• This PR changes the initialization (BackendApplicationContribution#initialize) of the localization service for plugins. With the proposed changes, the backend initialization does not wait for the cleaning of the localization cache folder.
• The PR also changes the return type of the initialize method from Promise<void> to MaybePromise<void> not to force downstream projects to use aync/Promise when doing the customization.

Why?

To speed up the backend startup when the Theia-based app is stated with multiple languages.

Originally from here:

I am seeing slow startup time with multiple languages. It's more than a sec.

2022-11-09T11:46:27.180Z root WARN Backend HostedPluginLocalizationService.initialize took longer than the expected maximum 50 milliseconds: 1070.7 ms [Finished 4.488 s after backend start]

Could something like this also work in Theia?

initialize(): void {
	this.getLocalizationCacheDir().then((cacheDir) => fs.emptyDir(cacheDir)).then(() => this._ready.resolve())
}

With the proposed change, the initialization is significantly less.

2022-11-09T12:05:46.544Z root INFO Backend HostedPluginLocalizationService.initialize: 3.0 ms [Finished 2.557 s after backend start]

Thank you!

How to test

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Alright, that looks good to me 👍