Nested Set Keystone field type

Add nested set field for Keystone headless CNS. It allows you to create and manipulate with tree-like structures with child and parent items.

Nested Set model

The Nested Set model is described here:
https://en.wikipedia.org/wiki/Nested_set_model

This module stores nested set data in the database using field names:
left, rigth, depth

Graphql Scheme

#### Output

- left: Int
- right: Int
- depth: Int
- weight: Int // Serial number through current branch
- parentId: ID// Keystone id of direct parent element
- isLeaf: Boolean, true if have no children
- childrenCount // counts children

#### Filters

- parentId: type ID // Keystone id of direct parent element
- childOf // filters all elements that are children of that Keystone id
- parentOf // filters all elements that are parent of that Keystone id
- prevSiblingOf: ID! // left sibling element, null if absent
- nextSiblingOf: ID! // right sibling element, null if absent

#### Mutation

##### Create and Update

- parentId: ID // Keystone id of direct parent element
- prevSiblingId: ID // Keystone id of left sibling element
- nextSiblingId: ID // Keystone id of right sibling element

##### Delete

If node has children they move to the parent of deleted node.

Recompiling

The module requires to be compiled with the same major versions of the NPM modules:
- keystone-6/core
- @keystone-ui/core
- @keystone-ui/fields
- graphql

So, if you get problems on launching Keystone with this module, getting these errors:

- Schema must contain uniquely named types but contains multiple types named "OrderDirection".
- Duplicate "graphql" modules cannot be used at the same time

You need to change dependencies to the exact ones, that is uses in your application and in the keystone-6/core package.

$3

1. Create a link to the sources of both projects - execute this command on each directory:
``
yarn link
`

2. In your project directory: cleanup the node_modules folder, link the Nested Set module, and preconstruct:

`
rm -rf ./node-modules && yarn link && yarn link keystone-field-nested-set && yarn preconstruct && yarn install
`
3. In keystone-nested-set directory: build with linked Keystone sources:
`
rm -rf ./node-modules && yarn link @keystone-6/core && yarn && yarn build
`

4. On your project directory: build with recently rebuilt Nested Set module:
`
yarn build
``