Add reference navigation and completion for `instance` and `method` attributes in cron.xml

[VitaliyBoyko]

We need to add reference and completion to the instance and the method attributes.

Expected result (*)

1. Completion and reference is added
2. Functionality is covered by tests

[VitaliyBoyko]

The reference for the instance tag seems to be working.

[drpayyne]

@magento I am working on this

[drpayyne]

Hi @VitaliyBoyko, looks like a couple PRs were made without referencing this issue. Hence this can be closed

• [212]_add reference navigation and completion for crontab.xml #280
• Added test coverage for references and completions of crontab instance and method #310

[VitaliyBoyko]

Thanks. Sure