This enables the editor directory to be linted, and fixes the remaining issues raised by ESLint. Various rules were fixed here including, no-shadow, no-undef, no-unused-vars and others.
I've generally gone conservative, disabling rules where it doesn't make sense to fix them (e.g. sometimes suggests use-services for tests, but it is only used once, or within a Chrome script).
Depends on D5585
Differential Revision: https://phabricator.services.mozilla.com/D5587
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extra : moz-landing-system : lando
Note that this patch also replaces legacy VK_* with KEY_*, and replaces
synthesizeKey() for inputting some characters with sendString() because
it's better and clearer what it does and it sets shiftKey state properly.
MozReview-Commit-ID: De4enbjux3T
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extra : rebase_source : 2296b84bff8e22f01eeb48cd8614fac5db11136a