feat: add header button

[bartlomiej-przymus]

1️⃣ Is this something that is wanted/needed? Did you create an issue / discussion about it first?

Yes I did. Related Issue: #34

2️⃣ Does it contain multiple, unrelated changes? Please separate the PRs out.

Nope.

3️⃣ Does it include tests, if possible? (Not a deal-breaker, just a nice-to-have)

Yes, I have Included test to check new functionality

4️⃣ Please include a thorough description of the improvement and reasons why it's useful.

This PR adds new Headers dropdown button to allow users to set headers in edited text:

This PR introduces new enum ToolbarButton::Header that can be used inside toolbarButtons([]) method to add new button to toolbar.

We can also control available header sizes by passing array of header sizes to new setHeaders() method.

QuillEditor::make('value')
    ->toolbarButtons([
        ToolbarButton::Header,
    ])
    ->setHeaders(['1', '2', false]),

will produce following:

The false in array stands for Normal text.

5️⃣ Thanks for contributing! 🙌

[makkinga]

I need this functionality in my project.

@rawilk can this please be implemented?