[BUGFIX] Initialize CodeMirror if element becomes visible to client

CodeMirror initialization fails if its textarea is not rendered yet. To
fix this issue, an Intersection Observer is installed and initializes the
CodeMirror instance when the element becomes visibile to the client.

Resolves: #90525
Releases: master, 9.5
Change-Id: I93247a41827547abec5369e5904ec4dd2354eb7e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63504


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christian Eßl <indy.essl@gmail.com>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>