Implemented parsing (merge to dev)

[hrszpuk]

Related issues

• Create tests for the parser #7
• Set up AST and Nodes for the feature set #8
• closes Implement parsing to fulfil test cases #9

Checklist

• Parsing expressions
• Parsing variable declarations
• Parsing variable assignments
• Parsing function calls
• Parsing while loop
• Parsing if-else
• Parsing for loop