* The `!has[tags]` filter didn't work because "tags" is an array
The negated `has` filter only considered empty strings, but not empty
arrays (such as the `tags` field).
* Add tests for `has` filter operator with array-like fields (tags, list)
* 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
List fields (such as tags) when evaluated to produce tiddlers result in empty arrays. Using the exact not equals, an empty array is not the same as an empty string. By using equivelent not equals, we state that the field is either != "" or anything that can be coerced to "". Which, based on https://dorey.github.io/JavaScript-Equality-Table/ is `false` `0` `[]` or `[[]]`` neither `false` nor `0` can be set as a tiddler field as both will end up being quoted (`"false"`, `"0"`) so this should work.
