Allow generic upsert conflict resolutions

[gwynne]

These changes are now available in 3.35.0

It is now possible to specify an SQLInsert's conflictStrategy as a generic SQLExpression, using the new genericConflictStrategy property, around which conflictStrategy is now a wrapper. All existing code will continue to work unchanged. This is intended for use by extensions to SQLKit which, e.g. provide access to database-specific conflict resolution syntax.

Also updates the docs to resolve lots of warnings and some broken links.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c0ea243) to head (951845f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #197   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          100       100           
  Lines         2606      2608    +2     
=========================================
+ Hits          2606      2608    +2     

Files with missing lines	Coverage Δ
...ers/Implementations/SQLConflictUpdateBuilder.swift	100.00% <100.00%> (ø)
...s/Prototypes/SQLCommonTableExpressionBuilder.swift	100.00% <ø> (ø)
...it/Builders/Prototypes/SQLCommonUnionBuilder.swift	100.00% <ø> (ø)
...ssions/Clauses/SQLConflictResolutionStrategy.swift	100.00% <100.00%> (ø)
Sources/SQLKit/Expressions/Queries/SQLInsert.swift	100.00% <100.00%> (ø)

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