refactor: get rid of Simplebar (vanilla js based scrollbar)

[vanbasten17]

This PR include this other one PR#2903

Work done so far

1. Getting rid of scrollbar webchat property customization: We don't want to support this feature anymore. This was done before in order to have "nice" styles in Internet Explorer and similar browsers. As IE is not supported anymore, we can remove this.
2. Having ids for relevant botonic containers: In this way we can have a better way to access the values of these well differentiated areas of the webchat.
3. Ensuring elements within webchat are spaced correctly: This is, spacing elements between in webchat for every container to push the other siblings with same distances.
4. Remove Simplebar in favour of a new component created by us: Simplebar was giving lots of headaches to us, so we can change it by containers with the ids created in point 2.
5. Fix replies container not scrolling
6. chatArea container to englobe messages list and replies containers
7. Recalculate chatarea size when iOS safari virtual keyboard is shown: As we have modified the containers which where important to calculate when the webchat has to be resized in iOS, we needed to adapt and update a little bit this logic.

Current state

As per the list described above, the functionality should be the same as before. The important thing to notice here is that WE DON'T RELY ANYMORE on simplebar selectors '.simplebar-content' and '.simplebar-content-wrapper', so bots using them should be adapted.

Next steps

• Create reusable hooks to do calculations over DOM elements or use Resizer observer API to have the code up to date with new standards and good code practices.
• Remove related logic not needed anymore after removing simplebar dependency (possibly tricky parts of the code related to ShadowDOM).

[github-actions[bot]]

Test Results

1 tests   1 :white_check_mark:  2s :stopwatch: 1 suites  0 :zzz: 1 files    0 :x:

Results for commit 64b9c00f.

:recycle: This comment has been updated with latest results.