Fixes#4082
This version removes selective updating of the tag index, instead completely clearing the index on each update. I'm investigating restoring that optimisation.
The test rig previously used a simplified implementation of shadow tiddlers which broke with the new indexing engine. There was also a problem that made that even if indexers were disabled they were still initialised.
This PR fixes both problems, in preparation for fixing #4082
* First pass at modular wiki indexes
An exploratory experiment
* Fix tests
* Faster checking for existence of index methods
We don't really need to check the type
* Use the index for the has operator
* Fix typo
* Move iterator index methods into indexer modules
Now boot.js doesn't know the core indexers
* Fix up the other iterator index functions
* Fix crash with missing index branch
* Limit the field indexer to values less than 128 characters
* Fallback to the old manual scan if the index method returns null
* Sadly, we can no longe re-use the field indexer to accelerate the `has` operator, because the index now omits tiddlers that have field values longer than the limit
Still need to make the index configuration exposed somehow
* Rearrange tests so that we can test with and without indexers
We also need to expose the list of enabled indexers as a config option
* Test the field indexer with different length fields
So that we test the indexed and non-indexed codepaths
