Add Function to convert data types to their display value

[dnlbauer]

Querying for metadata where only the "display" version is currently complicated.

Consider these three metadata fields:

(author:: Jon Doe)
(author:: [[Jon Doe]])
(author:: [Jon Doe](https://example.com))

The query

TABLE author
FLATTEN author
WHERE author = "Jon Doe"

only results in the first match:

File	Author
test	Jon Doe

The only query I found that matched all three cases was this one:

TABLE author
FLATTEN author
WHERE contains(string(author), "Jon Doe")

File	Author
test	Jon Doe
test	Jon Doe
test	Jon Doe

However, this expression is error-prone and quite complicated. This PR introduces a new utility function display() that tries to convert all objects into a valid string representation while preserving the display value. This means [[Jon Doe]] is converted to "Jon Doe" and [[Jon Doe|Joe]] is converted to "Jon", etc. The above query reduces to this much simpler expression:

TABLE author
FLATTEN author
WHERE display(author) = "Jon Doe"