add all Headings to every List-Item

[SpocWeb]

In Response to #2290 and #2325 I have prepared this minimal Prototype.
@blacksmithgu Please consider this as a basis for discussion, I am not pushing for this exact Design,
but including the Heading Hierarchy would increase the Expressivity very much.

This adds a new List to every ListItem containing the path of containing Headings in strictly monotonously decreasing Levels.
This forms a kind of xPath or jsonPath to each List Item
and thus helps to organize multiple Lists within the same Markdown Document in markdown.ts:

export class ListItem {
    symbol: string;
    link: Link;
    section: Link;
    /** List of all Headings 'above' this one in ascending Order, to be able to properly group List Items */
    headings: HeadingCache[];
    ...

I am unsure on how to handle the serialize(cache: ListSerializationCache): SListItem-Operation in PageMetadada.tx (line 260)
It does not seem to support the Obsidian HeadingCache Class, so I am converting it to a List of Heading-Strings
with proper number of #s to indicate the correct Heading Level
headings: this.headings.map(h => "#".repeat(h.level) + " " + h.heading),

Best Regards and thank you for this powerful component. I am aware that there are many factors to consider for the evolution of the DataView.