fix: `isSubmitting` flashes `false` during form submission

[MonsterDeveloper]

Fixes #101

Description

I described the issue in #101. This PR is my attempt to fix the bug.

I added a separate state variable to keep track of whether we're actually submitting the form through the network. It is activated when onSubmit is called, and is de-activated when the fetcher or navigation state (through the isSubmittingForm variable) are idle.

Then in the hook return we set isSubmitting as isSubmittingNetwork || methods.formState.isSubmitting, which means "actually submitting the form through the network or the internal react-hook-form hook is doing it's part."

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• [x] Unit tests

Checklist:

• [x] My code follows the guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation N/A
• [x] My changes generate no new warnings or errors
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules N/A

[MonsterDeveloper]

@AlemTuzlak I've been trying to write tests but didn't manage... I'm not very experienced with unit testing React hooks, maybe you can come up with a solution. For now it seems like the useMemo is not triggered when the mocked value of useNavigation, so the useEffect is never triggered...

[MonsterDeveloper]

False alarm, managed to get it working. Added a test case that fails when using current version of the hook and passes on the new one.

[MonsterDeveloper]

@AlemTuzlak do you mind taking a look, please?

Hvala:)

[MonsterDeveloper]

Mind doing some minor changes?

Done!