[FEAT] Refactor code to support diff methods of storing the scheduling info, and diff SR algorithms

[ronzulu]

The current code has a structure tied to the method of storing scheduling info within the note markdown file itself.

Refactoring the code is a prerequisite to implementing other methods, e.g. as requested in:
[FEAT] Stop using YAML; Move plugin info and data to separate file

[pikatwinky]

It's only my opinion but,

Talking about the note review mode... i think having the note schedulling info in the frontmatter is very useful when using combined with Dataview which allows you create, through the use of queries, custom review sessions to filter down only certain sets of notes based on properties (in particular using the 'due date' field), functionality none existent currently in the plugin for this mode (you have to review your notes in a global way), so if #162 is implemented, this alternative won't be possible anymore (until the feat be implemented).
On the other hand, talking about the flashcards, i think having their scheduling info in a separate db could help to unclutter the notes, but of course, maybe this last thing is unfeasible

Edit:
Sorry, I wasn't aware of this fork and #748.
I like the idea of having a setting for selecting what kind of storage to use.

[YIRU69]

I hope you can bring FSRS functionality into this project!

[ronzulu]

I hope so! I've completed implementing the re-factoring addressed by this issue, but not yet completed testing.

After this then there are a couple of other nontrivial tasks to complete...

[YIRU69]

Hi! Can you tell me approximately how long until I can use FSRS?

[ronzulu]

Hi @YIRU69 if I'm the one implementing, I'd think realistically say 6 months, so around October this year.