Remarkup: make less internal links open in new tabs

Summary:
This is an attempt to improve the default behavior in Remarkup about
links. It does not change any behaviors manually specified in the engine
and it does not change any behaviors related to external domains.

As default, now these kind of links will open in the same tab:

• anchors
• relative URLs
• absolute URLs pointing to the base-URI domain

All the other cases are kept as before - so they open in another tab.

In short, assuming you are we.phorge.it, here the changes:

This situation can further improve but it already covers most of the
cases where most users do not expect to break their navigation into
several tabs. Moreover, if an user wants to open a link in another
window, no one prevents from using the middle mouse button,
or CTRL+click or any other nice really basic feature from their browser.

Also, this change introduces a new CSS class, allowing web designers
to style these external resources.

Example CSS rule to try:

.remarkup-link-ext::before {
    content: "[external] ";
}

Closes T15161
Closes T15182

Test Plan:

• Copy the example text from this Task: https://we.phorge.it/T15161
• Verify that "internal resources" are internal links as default now
• Verify that "external resources" are still external links as before

Reviewers: O1 Blessed Committers, Cigaryno, avivey, speck

Reviewed By: O1 Blessed Committers, Cigaryno, speck

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15182, T15161

Differential Revision: https://we.phorge.it/D25118