[observer] dom-text-mapper and dom-text-matcher updates + migration

 * Update to dom-text-mapper a9d203ac (observer-5 branch)
 * dom-text-mapper's API has changed; see https://github.com/hypothesis/dom-text-mapper/commit/a9d203acc20041e37315fc9b584af309d02a8439 for details
 * Followed api changes in the impacted plugins:
   DomTextMapper, TextAnchors, FuzzyTextAnchors
 * On the `Anchor` abstract class, the `_createHighlight()` method is now
   asynchronous. (Done the migration in TextRangeAnchor and TextPositionAnchor.)
 * Updated dom-text-matcher to 940a06a4 (master branch)
 * Followed the API change described at https://github.com/hypothesis/dom-text-matcher/commit/940a06a4f72cde7e14f4e691cf1b926f0f74cc6d
 * Small, cosmetic changes

Fixes #45