Wrap content insertion in hooks

[daun]

Description

• Create new hooks content:insert and content:remove
• Allow hooking before insertion of containers into DOM and manipulating them
• Useful for debugging or adding styles, CSS variables, etc.
• Tested in playground
• Includes version dump and changelog update

Checks

• [x] The PR is submitted to the master branch
• [x] The code was linted before pushing (npm run lint)
• [x] All tests are passing (npm run test)
• [ ] New or updated tests are included
• [x] The documentation was updated as required

Additional information

Closes #11

[daun]

Having content: in there is important to understand that this is analogous to and replaces swup's own content: hook. Instead of replacing (content:replace), we're now first inserting (content:insert) and then removing (content:remove).

Same as content:focus and content:announce for a11y plugin. I'd rather go for self-explaining names than knowing which plugin triggered this exactly.

[hirasso]

Ah, didn't catch that content:replace is actually being replaced! Good to go then :)