Rename Node classes related to variables

[fabpot]

Whenever I work on Twig internals, it's always complicated to reason about variable names, probably because the class names are confusing. This PR is an attempt to find "better" and more explicit names.

This PR does the following renaming:

• NameExpression to Variable\ContextVariable Represents the value of a context variable like $context[VAR] ?? null

• AssignNameExpression to Variable\AssignContextVariable Represents a context variable assignment like in $context[VAR] =

• TempNameExpression to Variable\LocalVariable Represents a "private" local variable like $_l111