Add detailed Reader screen tracking

[kean]

Closes CMM-1171: Make sure that discover sub-feeds views are showing up in tracks as events.

This PR implements the "Structured Analytics" proposal. It's an MVP, so the implementation might be a bit convoluted, but it should get the job done.

Testing

The easier way to test it right now is to filter the Xcode logs with something like "tracked:". Here's the log with one of my testing sessions (new events marked with a blue marker).

⚪️ Tracked: reader_accessed <>
🔵 Tracked: screen_shown <screen: reader_discover, discover_tab: freshly_presed, source_immediate: reader_sidebar, source_origin: reader_sidebar, source_path: reader_sidebar, trigger_action: tap, trigger_component: sidebar>
🔵 Tracked: reader_discover_tab_shown <channel: freshly_presed, source: discover>
⚪️ Tracked: reader_freshly_pressed_loaded <site: , source: reader>
⚪️ Tracked: reader_discover_channel_selected <channel: recommended>
🔵 Tracked: reader_discover_tab_shown <channel: recommended, source: discover>
⚪️ Tracked: reader_discover_shown <list: Discover, source: reader, subscription_count: 10>
⚪️ Tracked: traintracks_render <fetch_algo: read:recommendations:posts/discover-stream, fetch_position: 1, railcar: nQ50XxvqYGu^, rec_blog_id: 18264223, rec_post_id: 3452>
⚪️ Tracked: traintracks_render <fetch_algo: read:recommendations:posts/discover-stream, fetch_position: 2, railcar: i8O$QLvdzPvO, rec_blog_id: 105434334, rec_post_id: 93751>
⚪️ Tracked: reader_discover_content_presented <>
⚪️ Tracked: reader_post_card_tapped <list: Discover, source: reader, subscription_count: 10>
⚪️ Tracked: reader_article_opened <blog_id: 105434334, is_jetpack: false, offline_view: no, post_detail_type: normal, post_id: 93751, reading_preferences_color_scheme: default, reading_preferences_font: sans, reading_preferences_font_size: normal, reading_preferences_is_default: true>
🔵 Tracked: screen_shown <screen: reader_article, source_immediate: reader_discover, source_origin: reader_sidebar, source_path: reader_sidebar > reader_discover, trigger_action: tap, trigger_component: post_card, trigger_position: 1>
⚪️ Tracked: reader_article_comments_opened <blog_id: 105434334, post_id: 93751, source: reader_post_details, subscription_count: 10>
🔵 Tracked: screen_shown <screen: reader_comments, source_immediate: reader_article, source_origin: reader_sidebar, source_path: reader_sidebar > reader_discover > reader_article, trigger_action: tap_comment, trigger_component: toolbar>

Key points:

• Tracks what tab is shown (reader_discover_tab_shown). Previously, we only tracked when you switched tabs.
• When you open an article, it contains information about what triggered it. If it's Discover, it also includes the selected tab (the main point of the linked ticket)

[dangermattic]

	1 Warning
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.

	1 Message
📖	This PR is still a Draft: some checks will be skipped.

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	31335
	Version	PR #25280
	Bundle ID	org.wordpress.alpha
	Commit	c3e78cc
	Installation URL	6322qe7enhf6o

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📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	31335
	Version	PR #25280
	Bundle ID	com.jetpack.alpha
	Commit	c3e78cc
	Installation URL	2hb1n9drsjrcg

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🤖 Build Failure Analysis

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[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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