Make an migration converter for the link-it field

[lenvanessen]

Schematic recently added the option for extension developers to write a custom converter, allowing fields like link-it and supertable to be imported/exporter with schematic

Could you also add this to link-it? Would greatly help manage multiple environments.

Example: https://github.com/verbb/super-table/commit/5188c999f4744110fe6eaa5744fb0b4c95e1bac2