Add a couple of interpolations for SQLQueryString that take Fluent keypaths

[gwynne]

New interpolations:

"SELECT \(col: \FooModel.$bar) FROM \(FooModel.sqlTable)" as SQLQueryString
"""
INSERT INTO \(FooModel.sqlTable)
    (\(ident: \FooModel.$bar), \(ident: FooModel.$baz))
VALUES
    ()
""" as SQLQueryString

[MahdiBM]

Didn't remember you're already using package traits

[gwynne]

Yep, this package was my very first use of traits. Still my only one so far, in fact.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.29%. Comparing base (b50120e) to head (5b3a9bb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
+ Coverage   91.25%   91.29%   +0.03%     
==========================================
  Files          62       63       +1     
  Lines         949      953       +4     
==========================================
+ Hits          866      870       +4     
  Misses         83       83              

Files with missing lines	Coverage Δ
...rces/FluentKitExtras/OptionalPointerProperty.swift	65.90% <ø> (ø)
...es/FluentKitExtras/OptionalReferenceProperty.swift	86.00% <ø> (ø)
Sources/FluentKitExtras/PointerProperty.swift	81.33% <ø> (ø)
Sources/FluentKitExtras/ReferencesProperty.swift	86.91% <ø> (ø)
...xtras/Builders/SQLJoinBuilder+FluentKeypaths.swift	95.23% <100.00%> (ø)
...as/Expressions/SQLQueryString+FluentKeypaths.swift	100.00% <100.00%> (ø)
...as/SQLKitExtras/BuiltinExpressions/SQLColumn.swift	100.00% <100.00%> (ø)

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