* Removed `Template` class and unified script & template parsing logic. Templates are scripts, so they should be defined as a script. Unless there will be separate template infrastructure, templates should share as much code as possible with scripts.
* Removed ScriptParseException in favour for ElasticsearchParseException
* Moved TemplateQueryBuilder to lang-mustache module because this query is hard coded to work with mustache only
This commit moves template support out of the Search API to its own dedicated Search Template API in the lang-mustache module. It provides a new SearchTemplateAction that can be used to render templates before it gets delegated to the usual Search API. The current REST endpoint are identical, but the Render Search Template endpoint now uses the same Search Template API with a new "simulate" option. When this option is enabled, the Search Template API only renders template and returns immediatly, without executing the search.
Closes#17906
Previously multiple extensions could be provided, however, this can lead
to confusion with on-disk scripts (ie, "foo.js" and "foo.javascript")
having different content. Only a single extension is now supported.
The only language currently supporting multiple extensions was the
Javascript engine ("js" and "javascript"). It now only supports the
`.js` extension.
Relates to #10598
