Fix: include custom option within Manual Donation levels select element

[JoshuaHungDinh]

Resolves GIVE-1290

Description

This PR updates the is_custom_price_mode to include v3 donation forms in its conditional check. This will enable the "Custom" option inside v3 Donation levels select element.

Acceptance criteria

When I want to add a manual donation for a custom amount on a Visual Builder form, I can select Custom from the Donation Level drop-down before entering the custom amount under Donation Amount, so I am sure that the correct amount will be saved.

Affects

Custom price modes
Manual Donations - donation level options

Visuals

Testing v3

custom-option-donation-levels.mov

Testing v2

Testing-v2-forms.mov

Testing Instructions

v3

• Install & activate Manual Donations add-on.
• Create a v3 donation form & enable donation levels.
• Go to donations & add a manual donation.
• Select the v3 form from the dropdown.
• Verify within the donation levels that custom can be chosen.
• Testing v3 forms without Donation Levels will require ( https://github.com/impress-org/give-manual-donations/pull/181 )

v2

• Create a v2 form enable custom amount
• Go to donations & add a manual donation.
• Select the v2 form from the dropdown.
• Verify within the donation levels that custom can be chosen.
• Disable custom amount through v2 form editing.
• Verify within the donation levels that custom is not an option.

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Self Review of code and UX completed

[JoshuaHungDinh]

Declined task.
Reference: https://stellarwp.atlassian.net/browse/GIVE-1290