[Feature]: Index custom tags

[thombruce]

Feature request

Custom tags are those tags where the user provides the key and value, separated by a colon (e.g. due:soon).

These were not indexed as part of the work on #98 because of their additional complexity compared with other tag types (also because Dexie can't index an array of objects in any way).

There is some discussion about this on PR #100 and on issue #97.

The idea is to introduce a new class and table - Tag/tags - which makes indexable/searchable/sortable these values:

• todoID
• key
• value

Later, it should also be capable of handling a type property (see discussion of timer[t]:1h23m-like tags on #97).

While this is just for custom tags initially, it may be reused for project, context and hashtag style tags... though only if this appears to be useful (right now they have all the utility they need stored directly alongside their todos).

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