way2js4 - Messages and MathEvaluate

[peterdell]

• Replace iError() with Error(),
• Introduce IEvaluationContext to reduce direct dependencies and enable unit tests for MathEvaluate
• Use const and translated comments
• Apply Lazarus Standard formatting for reworked parts

Besides:

• Update HOW-TO documentation for Python tests. Now, the test suite also works on Windows.