Add returning clause support

[cerredz]

Closes #8.

Adds a query-level Returning(...) / ReturningRaw(...) API for insert and update queries, with compiler placement hooks so engines can render dialect-specific output clauses.

Implemented dialects:

• PostgreSQL: appends RETURNING ... after insert/update statements.
• SQL Server: emits OUTPUT INSERTED... in the correct position before the insert source or update WHERE clause.

Unsupported compilers now fail fast if a returning clause is compiled instead of silently generating invalid SQL.

Verification:

• dotnet test QueryBuilder.Tests\QueryBuilder.Tests.csproj --filter FullyQualifiedName~InsertTests|FullyQualifiedName~UpdateTests
• dotnet test QueryBuilder.Tests\QueryBuilder.Tests.csproj --filter FullyQualifiedName!~MySqlExecutionTest
• Full unfiltered test run reaches 336 passing / 1 skipped before failing only the MySQL integration fixture because this environment has no MySQL host configured.
• git diff --check