feat: AST generation for compiler builtins convDate and isZero

[LeanSerra]

Description

This PR adds AST generation for convDate and isZero

For convDate

• convDate: is implemented by getting the date: DD-MM-YYYY we have to convert it to YYYYMMDD
  • If we treat each part of the date as an integer
    • YYYY * 1000 will give us: YYYY0000
    • MM * 100 will give us: MM00
    • DD * 1 will give us DD
• Adding up all these values will give us YYYYMMDD

For isZero

• isZero is implmented by after generating the AST for the expression inside isZero creating a leaf with a 0 value and creating a node with comparison "==" with the expression as the left_child and the 0 as the right_child

[MatiBarcia]

LGTM