Add parsing for element_sections in LongClass

[AntoineGautier]

Currently the parser only handles element_list in LongClass:

export class LongClass extends Element {
// ...
    this.elementList =
      specifier.composition.element_list
        ?.map((e: any) => {
//...

However, if elements are declared in a public or protected section, they are not included in element_list but in element_sections.public_element_list or element_sections.protected_element_list.

This ticket is to add support for parsing these sections in the LongClass constructor.