luci-base: reimplement the add method of CBIJSONConfig

[emzet]

This PR fixes a bug in the calculation of section_id in the add method of CBIJSONConfig when using nested sections (subsections).

When using the JSONMap, data are retrieved via a JSON file or a custom JS object. Internally, the object is converted into a UCI-like format. Its top-level keys are treated as UCI section types, while the object or array-of-object values are treated as section content.

const data = {
  "first_section": [
    {
      "title": "my first section first item"
    },
    {
      "title": "my first section second item"
    },
  ],
  "second_section2": [
    {
      "title": "my second section first item"
    }
  ]
};

Section types are stored in the main data object with key:value pairs (just plain JS object). Keys are generated during init method and stored in a flat structure. Indexing increments globally, regardless of the section type.

{
  "first_section0": {
    "title": "my first section first item",
    ".name": "first_section0",
    ".type": "first_section",
    ".anonymous": true,
    ".index": 0
  },
  "first_section1": {
    "title": "my first section second item",
    ".name": "first_section1",
    ".type": "first_section",
    ".anonymous": true,
    ".index": 1
  },
  "second_section2": {
    "title": "my second section first item",
    ".name": "second_section2",
    ".type": "second_section",
    ".anonymous": true,
    ".index": 2
  }
}

The problem in the original code occurred when using nested sections, e.g., a GridSection inside another GridSection. In this case, adding a new section (via the UI "add" button) would compute the wrong section_id, causing it to always return the last section instead of creating a new blank one.

const formMap = new form.JSONMap(data);
const section = formMap.section(form.GridSection, 'first_section');

section.tab('tab1', _('Tab 1'));
section.tab('tab2', _('Tab 2'));

option = section.taboption('tab1', form.Value, 'title', _('Title in parent Grid'));
option = section.taboption('tab2', form.SectionValue, '_title', form.GridSection, 'second_section');

const subsection = option.subsection;

option = subsection.option(form.Value, 'title', _('Title in nested Grid'));

Changes in this PR fixes this problem:

• renamed next_index to max_index to make its purpose clearer
• next_index is now computed by incrementing the max_index found in data, regardless of section type
• removed num_sections_type as it is no longer used
• the next_index value is now used both for creating the section_id and as the value of .index, ensuring uniqueness