feat: add removeInsertedCSS

[brenca]

Description of Change

Fixes #16475.
This PR adds a pair for insertCSS, that kinda works like setTimeout and clearTimeout - insertCSS has a return value that we didn't use before, that is a unique key for the inserted CSS. Using that, we can remove the inserted CSS.

Depends on #18792

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Added removeInsertedCSS() to webFrame / webContents / <webview>.

[jkleinsc]

Overall looks good to me - just a small nit on the docs.

Also, can you rebase the latest from master onto this PR? It should resolve the failing CI.

[brenca]

@jkleinsc The one remaining failure seems unrelated, ready for review again!

[jkleinsc]

@brenca CI is green. Just need to rebase and we can merge this one.

[MarshallOfSound]

@brenca This PR is in conflict, could you rebase and fix lint so we can look at merging this 👍

[ckerr]

@brenca, is this PR still active? It hasn't been touched in a month; however, last time it had activity it looks like all it needed was a rebase/delint before it could land

[miniak]

@brenca please rebase

[miniak]

@ckerr I've rebased and fixed the PR. I've separated a prerequisite change to #18792 (1st commit in this PR), which should be merged fist.