Support INDEX creation in import_from_json

[majdisorder]

Is your feature request related to a problem? Please describe.

It would be a nice addition if import_from_json had INDEX support. This will further the capabilities for users who wish to use import_from_json as a way to create a (empty) database instance.

Describe the solution you'd like

Add support for a new object with "type": "index" to import_from_json.
I guess it would also make sense to include indexes in export_to_json.

As an alternative, an "indexes": [] property could be introduced to the "type": "table" object.

Describe alternatives you've considered

It's now possible to tack on a CREATE INDEX at the end of your table's "sql" property. This is okay for single indexes but quickly becomes unreadable and unwieldy for multiple indexes or tables with many columns.

[{
	"name": "my_table",	
	"sql": "CREATE TABLE my_table (..., col_name TEXT); CREATE UNIQUE INDEX my_table_col_ix ON my_table(col_name);",
	"type": "table",
	"row_array":[]
}]

[2shady4u]

@majdisorder I will implement this!

[2shady4u]

Work ongoing on this branch:
https://github.com/2shady4u/godot-sqlite/tree/feature/add-indexes-to-json