[GROOVY-6623] Interpreter mode for groovysh, with caveats

Now works:

    groovy:000> def x = 3
    ===> 3
    groovy:000> x
    Unknown property: x
    groovy:000> :set interpreterMode true
    groovy:000> def x = 3
    ===> 3
    groovy:000> x
    ===> 3

Now fails (Type/method declaration on same line as statement):

    x = 3; class Foo{}
    x = 3; def foo() {}