Add wikify operator

[Jermolene]

This PR adds a "wikify" operator that has the same functionality as the wikify widget. I regard this as quite controversial because while it unlocks some important capabilities, it also suffers from performance issues and invites users to adopt patterns that can be dangerous:

• The performance issues arise from the fact that the wikify operator does not allow us to use the usual mechanism for caching parsed tiddlers. This means that to evaluate the operator always requires a full parse of the input text as well as the construction of a widget tree (which would always be required, even if caching were to be implemented somehow)
• The issue with dangerous patterns is that users might be tempted to replace complex filters with a wikitext equivalent, for example using the <$list> widget to generate a set of numbers instead of the [range[]] operator. Besides being horribly inefficient, there are pitfalls with unexpected wikification

This PR is cherry picked from #8702; I plan to refactor the work there to not require the wikify operator because of the performance issues.

The question now is what are the use cases for the wikify operator, and are there any other ways to accomplish those use cases?

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[Leilei332]

When updating the flexoki palette, I found that the tag won't display properly unless index background, foreground and tag-background 's value is hardcoded. So the wikify operator should be used in the tag macro.

But we had better not use it at present since tag pill is often used in UI, which may cause performance issues.