feat: add hierarchy levels for custom anchor nesting

[vmikk]

This PR adds support for explicit hierarchy levels in anchor tags, allowing users to create structured TOC-like hierarchies. Previously, only region tags (like SECTION) supported hierarchical nesting through their start/end pairs. Non-region anchors could only exist at the root level, which make impossible to create multi-level TOC structures with custom tags.

To specify hierarchy level for a particular anchor, user should add hierarchyLevel property to tag configuration. By default, non-region tags have hierarchyLevel of 1, and region tags maintain existing behavior.

Configuration example:

{
  "commentAnchors.tags.anchors": {
    "L1": {
      "scope": "workspace",
      "highlightColor": "#FF6B6B", 
      "behavior": "anchor",
      "hierarchyLevel": 1
    },
    "L2": {
      "scope": "workspace",
      "highlightColor": "#4ECDC4",
      "behavior": "anchor", 
      "hierarchyLevel": 2
    },
}

Test data:

// L1 First level header
function someFunction() {
    // L2 Nested under L1
    // L2 Another item under L1

}

// L1 Another first level header  
function anotherFunction() {
    // L2 Item under second L1
    // L2 Something nested
}

Which should results into:

No breaking changes were made to existing API or behavior.