Add DateDiff functions and SQL translation

[khellang]

Closes #3

[codecov]

Codecov Report

Merging #6 (403d257) into master (b94761a) will increase coverage by 13.56%.
The diff coverage is 94.11%.

@@             Coverage Diff             @@
##           master       #6       +/-   ##
===========================================
+ Coverage   76.35%   89.91%   +13.56%     
===========================================
  Files          15       16        +1     
  Lines         296      347       +51     
===========================================
+ Hits          226      312       +86     
+ Misses         70       35       -35     

Impacted Files	Coverage Δ
...ernal/SqliteNodaTimeDateDiffFunctionsTranslator.cs	93.87% <93.87%> (ø)
EFCore.Sqlite.NodaTime/Extensions/Utilities.cs	83.33% <100.00%> (+0.31%)	⬆️
...ternal/SqliteNodaTimeMethodCallTranslatorPlugin.cs	100.00% <100.00%> (ø)
...ructure/Internal/SqliteNodaTimeOptionsExtension.cs	63.63% <0.00%> (+4.54%)	⬆️
...ors/Internal/SqliteNodaTimeMethodCallTranslator.cs	95.50% <0.00%> (+14.60%)	⬆️
...slators/Internal/SqliteNodaTimeMemberTranslator.cs	93.33% <0.00%> (+40.00%)	⬆️
...SqliteNodaTimeEvaluatableExpressionFilterPlugin.cs	100.00% <0.00%> (+50.00%)	⬆️

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