RFR: 8340565: Create separate index page for terms defined by the index tag

[Hannes Wallnöfer]

This change adds a conditional `search-tags.html` page to list all search tags defined by the `{@index ... }` tag. Since search tags work very similar to the `{@systemProperty ...}` tag and are internally represented by the same class I changed `SystemPropertiesWriter.java` into an abstract base class called `IndexItemListWriter.java`, with concrete subclasses for system properties and search tags.

JavaDoc also generates index items for documentation headings, these are not included in `search-tags.html`.

[This is the Search Tags page for JDK API docs](https://cr.openjdk.org/~hannesw/8340565/api.00/search-tags.html) (top-level files only).

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Commit messages:
 - Remove trailing whitespace
 - Fix comment
 - JDK-8340565: Create separate index page for terms defined by the index tag

Changes: https://git.openjdk.org/jdk/pull/21429/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21429&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340565
  Stats: 531 lines in 13 files changed: 363 ins; 161 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/21429.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21429/head:pull/21429

PR: https://git.openjdk.org/jdk/pull/21429