qmlls: JS statement completion

Implement completion for all JS statements.

Always think about the two cases:

• completing the keyword (`for`, `let`, `switch`, for example)
• completing the statement itself (inside `for(let x = <complete here>; ; ){}`, for example).

Here a list: ( indicates merged, indicates 'wont do', needs to be done and in progress)

• BlockStatement
• VariableStatement
• EmptyStatement
• ExpressionStatement
• IfStatement
• IterationStatement
  • do {} while();
  • while() {}
  • for() {}
  • for (... in ...) {}
  • for (... of ...){}
• SwitchStatement switch() {}
• CaseBlock/CaseClauses/DefaultClause/CaseClause
• Continue/Break statement
• ReturnStatement
• WithStatement
• LabelledStatement
• ThrowStatement
• TryStatement
• Declaration (check again at https://262.ecma-international.org/7.0/#prod-Declaration)
  • Lexical Declaration (let and const)
  • class declaration
  • Hoistable declaration: function + function* (including lambdas)