Implement is_complete for the square task

[moon0326]

Changes proposed in this Pull Request:

This PR implements is_complete function for the Square task.

The logic is copied from Square plugin's is_connected function.

• This PR is a very minor change/addition and does not require testing instructions (if checked you can ignore/remove the next section).

How to test the changes in this Pull Request:

• Have you followed the Writing high-quality testing instructions guide?

1. Create a new Square PAO site.
2. SSH into the site and replace includes/tasks/class-wc-calypso-task-get-paid-with-square.php with the one included in this PR.
3. Go to WooCommerce -> Home and click on Get paid with Square task.
4. You can either sign up for Square or manually set wc_square_access_tokens option. I would recommend signing up a square account.
5. Once you're redirected back to the site from Square, the task should be marked as completed.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.

[github-actions]

Size Change: 0 B

Total Size: 201 kB

ℹ️ View Unchanged

Filename	Size
./build/53.js	1.08 kB
./build/index.css	878 B
./build/index.js	126 kB
./build/marketing.js	58.3 kB
./build/payment-gateway-suggestions.css	1.24 kB
./build/payment-gateway-suggestions.js	6.57 kB
./build/plugins.js	3.93 kB
./build/style-index.css	2.15 kB
./build/style-marketing.css	800 B

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[cpapazoglou]

LGTM!