[INTERNAL] lib/processors/jsdoc: implement ui5-experimental-since

 - add tag @ui5-experiemental-since, expecting a version but nothing
   else (no additional text)
 - @ui5-experimental-since must not be used together with @SInCE
   @experimental in the same JSDoc comment

Within metadata info objects and in JSDoc doclets, the `experimental`
property can either be a text, following the usual pattern
("As of x.y...") or a SemVer. It will be a SemVer if and only if it
stems from the new `ui5-experimental-since` tag.

In the resulting api.json file, `experimental`, if defined, is still
an object for compatibility reasons. If `ui5-experimental-since` was
used, only the `since` property of the object will be filled. As a
consequence, a consumer of api.json can't distinguish between the
following two usages:

  @experimental As of 1.139
  @ui5-experimental-since 1.139

Should this be needed, we could add an empty string for the first case.

Cherry-picked from UI5/openui5@daa87ca90

Author: codeworrior