Support optional initializer expression in VariableDecl convenience inititializer

[fwcd]

As proposed in #506 (comment), this is a small QoL change that extends the existing convenience initializer for VariableDecl to support optional initializer expressions (and also makes the type annotation optional):

// let x = y
VariableDecl(.let, name: "x", initializer: "y")

// var x: String = "abc"
VariableDecl(
  .var,
  name: "x",
  type: "String",
  initializer: StringLiteralExpr("abc")
)

[fwcd]

@swift-ci please test