Support deprecated strikethrough directly in the editor

[DucNgn]

Feature Description:

Would be nice if Theia highlights deprecated calls with strikethrough in the editor

• Similar to VSCode:
  

[vince-fugnitto]

I believe it will require an update to monaco v0.21.0 in order to make use of the showDeprecated flag:

https://microsoft.github.io/monaco-editor/api/interfaces/monaco.editor.ieditoroptions.html#showdeprecated

[vince-fugnitto]

The following should now be possible with the latest monaco upgrade.
The preference editor.showDeprecated is not yet supported at the moment (it does not display deprecations in the editor as is).

theia/packages/editor/src/browser/editor-preferences.ts

Lines 1078 to 1082 in f5d042f

	'editor.showDeprecated': {
	'description': 'Controls strikethrough deprecated variables.',
	'type': 'boolean',
	'default': true
	},