feat(legacy): add current date macro to string block criteria (#3013)

### Description

It is useful to be able to automatically filter tracks to a certain date
or day when scheduling tracks for playback. This adds the ability to do
just that in any string filter with the `date{format}` macro.

 Closes:  #3012
libretime/libretime@451652b

docs/user-manual/playlists.md

@@ -37,6 +37,7 @@ Smart blocks are automatically filled with media files from the LibreTime librar
 To create a smart block, click the **Smartblocks** button on the left sidebar, and select **New** from the toolbar. Like a playlist, smart blocks can have a title and description, which you can edit. This helps you find relevant smart blocks in searches.
 
 Fill out the smart block's **Name**, **Search Criteria**, and **Limit to** sections. The search criteria can be any one of LibreTime's metadata categories, such as **Title**, **Creator** or **Genre**. The modifier depends on whether the metadata in question contains letters or numbers. For example, **Title** has modifiers including _contains_ and _starts with_, whereas the modifiers for **BPM** include _is greater than_ and _is in the range_.
+To filter tracks using today's date information, use the `now{}` macro. Format characters are listed in the [php documentation](https://www.php.net/manual/en/datetime.format.php). For example, to filter to tracks with a **Title** that ends in `Instrumental Jan 2024` where `Jan 2024` is the current month and year, add a criteria for **Title** with a modifier of **ends with** and a value of `Instrumental now{M Y}`.
 
 If you have a large number of files which meet the criteria that you specify, you may wish to limit the duration of the smart block using the **Limit to** field, so that it fits within the show you have in mind. Select **hours**, **minutes** or **items** from the drop-down menu, and click the **Generate** button again, if it's a static smart block. Then click the **Save** button.