Register the lower and upper-case version of a function

[nils-braun]

This allows to call both

SELECT F(x) FROM data

as well as

SELECT f(x) FROM data

no matter how the function was registered

[codecov-commenter]

Codecov Report

Merging #177 (90adddf) into main (90569c9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #177   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           57        57           
  Lines         2196      2197    +1     
  Branches       299       299           
=========================================
+ Hits          2196      2197    +1     

Impacted Files	Coverage Δ
dask_sql/context.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 90569c9...90adddf. Read the comment docs.