Add a EvalExpr block to NEPO. Implement a typecheck visitor.

[rbudde]

A long time ago the ANTLR4 grammar 'Exprly.g4' was defined as a first attemt to realize a textual form of NEPO (as a compagnion to the visual Blockly). This grammar concentrates on the expressions of NEPO.

This can be used in an EvalExpr block, which stores an user-defined textual form of an expression as '3 + 4*(x>5 ? 6 : 4)'.
This string can be parsed and translated to an AST (class ExprlyVisitor.java).
Then the AST must be typechecked (TypecheckCommonLanguageVisitor.java).
After that simlation and code generation should work out-of-the-box.

The classes exist in a preliminary version and must be completed.
Then the worker/visitor-architecture must be used to attach the components to all existing plugins. Typechecking should be applied to the complete NEPO program (independent of the visual or textual syntax)

The implementation must be tested (unit + system) rigorously.

[rbudde]

duplicate of #1688