feat: Higher-level machine + Add conditional expression

[Azorlogh]

Back-porting of https://github.com/formancehq/stack/pull/293 to v1

Make machine higher-level: Programs are now structured as a high-level trees rather than bytecode. Also adds:

• Boolean type
• Arithmetic conditions (== != < <= > >=)
• Logical operations (! && ||)
• Conditionals through a ternary operator (syntax TBD, likely if $cond then $istrue else $isfalse)