[LS] Suggest additional completions

[turbolent]

Context

@MaxStalker created some great code snippets for VS Code. They're very convenient and we should provide them as a default in the language server, i.e. in the Visual Studio Code extension and the Playground, e.g.:

• New empty array literal
• Null-coalesce
• Fields + initializer
• Pre and post-condition
• If statement, if-else statement
• While loop
• For-in loop
• Optional binding, with and without else block

Definition of Done

• Port more code snippets as completion suggestions to the language server