fn:replace: Combine $replacement and $action parameters

[ChristianGruen]

We could combine the competing $replacement and $action parameters:

replace(
  'this is a test',
  '(\w)(\w+)?',
  fn($s, $g) { upper-case($g[1]) || lower-case($g[2]) }
)

Original comment: #1863 (comment)

[liamquin]

+1 to this.

[michaelhkay]

The text also need to be revised to make it clear that when there are multiple matches, the replacement string can be different for each match, because each match will (in general) have different captured groups.

PR #1856 makes significant changes to the text of fn:replace, so I propose to wait until that has been applied before raising a PR for this issue.