Further development of Fock states

[apkille]

• Adds a documentation page for using Fock states.
• Moved predefined Fock objects from predefined.jl to a new file predefined_fock.jl.
• Added displacement and phase shift operators, DisplaceOp and PhaseShiftOp, respectively.
• Added simplification rules and rewriter with tests for Fock states.
• Updated express functionality for Fock objects.

[apkille]

@krastanov you might disagree with the following changes, which is OK and we can discuss them:

• I replaced ContinuousCoherentState and DiscreteCoherentState with simply CoherentState, as that distinction didn't seem too important to include in this package, particularly since QuantumOptics.jl only has coherentstate.
• In express_nolookup, before for Fock states we would change the dimension of the Fock basis to 2 with the warning that the cutoff wasn't defined, but of course this isn't always the case, as we can define a finite cutoff. So, if the cutoff is infinite, then we change the dimension to 2.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.65%. Comparing base (f291b77) to head (3f9fe6d).

Files	Patch %	Lines
src/QSymbolicsBase/predefined_fock.jl	93.75%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #69 +/- ## ========================================== + Coverage 72.94% 74.65% +1.70% ========================================== Files 18 19 +1 Lines 791 789 -2 ========================================== + Hits 577 589 +12 + Misses 214 200 -14 ```

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[Krastanov]

let me know if I am missing something that is making the requests unreasonable

[apkille]

All of your requests sound good to me.

@krastanov Edit: actually, there's one small issue related to the first request, which I mentioned earlier in the first round of review:

The problem with raising a warning in the constructor is that it creates allocations and increases the runtime. This causes tests in test_express_opt.jl to not pass. I instead added a warning to the docs page

[Krastanov]

ah, noted! Sounds good, let's keep it as a warning to the docs for now

[apkille]

@krastanov copying this down below: in your commit you modified QuantumOpticsRepr so that it has optional arguments, yet in your review you said that it should have a keyword argument. Could you please clarify?

[Krastanov]

My bad. I meant kwarg. Maybe with @kwdef