Add support for json variables file

[jasle]

Robot Framework support using json files as variables file:
https://robotframework.org/robotframework/latest/RobotFrameworkUserGuide.html#variable-file-as-json

Currently this plugin shows a 'Import file not found' notice when importing a json variables file and displays 'Variable definition not found' on every variable from it.

It would be great when this plugin could support json variable files.

[Redysz]

I will just leave here an example for easier reconstruction:

*** Settings ***
Variables    my.json

*** Variables ***
${Bar}    Bar

*** Test Cases ***
Dummy Test
    Log To Console    Foo: ${Foo} and Bar: ${Bar}

and my.json file:

{
  "Foo": "Foo"
}