RichText: fix tree output after applying format

[ellatrix]

Description

Fixes #12314. Still needs some additional tests.

Problem: Applying a format around an existing format (greater selection), will chop the applied format into 2-3 pieces. Why? Because the format is just added at the end of each format array at every index:

     1111
111110000
link bold
     link

Instead, this should be:

     0000
111111111
link bold
     link

Solution: check the position at which to insert first. This should be the lowest value between the start and end indices.

How has this been tested?

Make something bold. Select the bolded text, including something more. Now link it. The link should not be split into two elements.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[draganescu]

I tested locally and it solves the problem as described. Reading the code I don't spot any problem.
Minor thing @ellatrix while the solution here solves the problem there is another inconsistent behavior:

• in a selection containing test text <b>bold item</b><em>italic!</em> applying a link results in <a href='#'>test text <b>bold item</b><em>italic!</em></a>
• YET in a selection containing only<b>bold item</b> applying a link results in <b><a href='#'>bold item</a></b>

However I am unsure if this is in the scope of this fix, so I say we merge this as it is.

[ellatrix]

@draganescu Thanks for reviewing! I don't see a problem there, what's wrong with the second case?

[draganescu]

@ellatrix it might not be a problem, the difference is that in the first case the link tags wrap the content and any tags, and in the second case the link tag is placed inside the outer tag. It just doesn't work the same, maybe it doesn't have to, I just noticed :) otherwise no issue.

[ellatrix]

Thanks @draganescu. I didn't really think much of it, but you have a point. Maybe the rule for applying tags should be: wrap as much content as possible. Currently it seems to be: wrap as little content as possible.