feat: Improve FormLayoutElement API and add FormItemElement

[TatuLund]

Fixes https://github.com/vaadin/testbench/issues/1886

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vaadin-bot]

Hi @TatuLund and @vursen, when i performed cherry-pick to this commit to 23.5, i have encountered the following issue. Can you take a look and pick it manually? Error Message: Error: Command failed: git cherry-pick 002f6ab445fbaa0fc139afb2b4f7db831777b583 error: could not apply 002f6ab445... feat: add FormItemElement and related FormLayoutElement methods (#7243) hint: After resolving the conflicts, mark them with hint: "git add/rm ", then run hint: "git cherry-pick --continue". hint: You can instead skip this commit with "git cherry-pick --skip". hint: To abort and get back to the state before "git cherry-pick", hint: run "git cherry-pick --abort".