scripts: gen_isr_tables: Break code into classes

This commit breaks the code into functional classes.
This way the functionality is visibly splitted into
functional parts and it is easier to replace the
specific parser part to implement new code generators.

There is also common functionality to handle multi level interrupts
moved to configuration class.

Signed-off-by: Radosław Koppel <radoslaw.koppel@nordicsemi.no>