Feature: Semantic validation

[martin-azpillaga]

Inspired by issue #1, this PR implements semantic validation for identifiers in the calc language.

Following the rules of most mainstream programming languages, identifiers need to be declared before usage, and cannot be declared multiple times.

In the case of calc, the editor will report errors when:

• More than one input statements declare the same identifier
  
• An identifier used in a calculation expression has not been declared in previous lines
  
• An identifier used in an output statement has not been declared in previous lines
  

Instead of an ANTLR listener or visitor, a simple parse tree walker is used to check the relevant nodes.
Additional validation tests have been added to the parsingTest file.
The features of the editor are now documented in the readme.