fix: make feature strings more robust to whitespace

[nornagon]

Fixes #15594

The algorithm was already slightly lenient about whitespace, but surprisingly so. This makes it ignore all whitespace around , and = (and at beginning/end of the feature string).

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• PR title follows semantic commit guidelines

Release Notes

Notes: be more lenient about whitespace in webview's "webpreferences" feature string

[release-clerk]

Release Notes Persisted

be more lenient about whitespace in webview's "webpreferences" feature string