Support editor lookup by GitHub username

[kfranqueiro]

This PR implements resolving GitHub usernames via the data-editor-github attribute, as an alternative to data-editor-id.

As this involves requests to GitHub's users API, the GH_TOKEN environment variable is checked for a token to pass.

Changes

• Adds new resolveGithubUsernameToId utility function to lib/rules/util.js, which resolves a GitHub username to a W3C ID
  • Edge cases: returns null if the GitHub username is not found in either system (404); throws an error if any other HTTP error occurs
  • Lookups that result in success or 404 are temporarily cached, to avoid redundant lookups in the case of multiple runs, or multiple affected rules within the same run
• Adds logic to rule modules headers/dl.js, headers/editor-participation.js, and metadata/editor-ids.js pertaining to GitHub username lookup; each of these checks for null return values and catches errors to report the appropriate error message
• Moves getEditorIDs logic from lib/validator.js to lib/rules/metadata/editor-ids.js, as this was the only place it was used, and this relegates all resolveGithubUsernameToId calls to rules modules
• Adds mention of data-editor-github to editorSection rule in lib/rules.json
• Adds localization strings for new error cases under each rule
  • These have been added individually to work well with sr.error within each rule module, although they have the same content; the thought was that the more critical sr.throw would be excessive
• Tests:
  • Adds mocks for both the GitHub and W3C API calls, including cases that resolve with 404 or 500 status
  • Adds success/missing/failure tests for the two headers-related rules, and a Basics test for the metadata resolution
• Misc:
  • Removes explicit target from tsconfig, which was set to an excessively-old value which caused an interop issue with octokit's typings (the default is esnext when module is nodenext)