refactor: small code style tweaks, add missing JSDoc

[web-padawan]

Description

Some small updates to date-picker and infinite-scroller extracted from #4770:

1. Replaced ES5 leftovers [].forEach.call and Array.prototype.slice.call with array spread,
2. Replaced legacy Polymer-specific root property coming from Polymer 1 era with shadowRoot,
3. Changed wording to use slot instead of insertionPoint (which was a Polymer 1 equivalent),
4. Removed mention of iron-media-query from CSS comment (we now use a different approach),
5. Added missing JSDoc annotations to all vaadin-infinite-scroller properties and methods.

Type of change

• Refactor

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[vaadin-bot]

Hi @web-padawan , this commit cannot be picked to 23.1 by this bot, can you take a look and pick it manually?
Error Message: Error: Command failed: git cherry-pick 834683c
error: could not apply 834683c... refactor: small code style tweaks, add missing JSDoc (#4777)
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".