Fix exported variables declaring or inferring the type were not converted correctly

[Maran23]

Fixes: #75852

Fixed the following two problems which may occur when converting exported variables:

• When an exported variable declared a type like the following:
  export(int) var integer: int = 2
  it was incorrectly converted to:
  @export var integer: int: int = 2
• When an exported variable inferred the type like the following:
  export(int) var integer := 2
  it was incorrectly converted to:
  @export var integer: int := 2

Added also a bunch of conversion tests.

[akien-mga]

Thanks!