Provide an option to require explicit `var` declarations

[christoph_schlumpf]

FYI :

Requiring `var` for variable declarations

We’re making a fairly major change in Mojo syntax leading up to 1.0: var will now be required before new variable declarations. Previously, you could declare a new variable either through the use of an explicit var variable = ... or through variable = .... We will start warning about the use of the latter in the next nightly, and then make it an error to use that syntax before 1.0. The two ways of declaring a new variable in Mojo have been the subject of lengthy prior discussions and we know th…