Refactor SharedVariable and its interfaces

[brandonwillard]

This PR contains the following changes to SharedVariable and related machinery:

• Use singledispatch to register type constructors
  • This removes some unnecessary redundancy and complexity.
• Make SharedVariable.default_update a fixed property that defaults to None
  • This makes the interface clearer and its usage simpler.
• Automatically perform Type checking and conversion when assigning to SharedVariable.default_update
  • This will help identify updates-related problems much earlier.
• Misc. docstring and error message updates

[codecov]

Codecov Report

Merging #1305 (65c6faf) into main (5aa1780) will increase coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1305      +/-   ##
==========================================
+ Coverage   74.10%   74.12%   +0.02%     
==========================================
  Files         174      174              
  Lines       48681    48660      -21     
  Branches    10379    10372       -7     
==========================================
- Hits        36074    36071       -3     
+ Misses      10313    10297      -16     
+ Partials     2294     2292       -2     

Impacted Files	Coverage Δ
aesara/compile/function/pfunc.py	84.42% <100.00%> (+2.01%)	⬆️
aesara/compile/sharedvalue.py	93.75% <100.00%> (+13.26%)	⬆️
aesara/scan/basic.py	84.59% <100.00%> (ø)
aesara/sparse/sharedvar.py	94.11% <100.00%> (ø)
aesara/tensor/random/var.py	95.45% <100.00%> (-4.55%)	⬇️
aesara/tensor/sharedvar.py	82.60% <100.00%> (+5.05%)	⬆️