Simplify and modernize code style configuration

[craigsmitham]

Summary

Closes #55

• Phase 1: Remove ~50 redundant .editorconfig settings matching Roslyn defaults (formatting rules, parentheses preferences, default-value code style options). Replace ~35 individual :suggestion severities with a single dotnet_analyzer_diagnostic.category-Style.severity = suggestion bulk line, keeping only targeted overrides for warning and silent rules.
• Phase 2: Add 8 modern C# 10-13 style rules: primary constructors, collection expressions, extended property patterns, readonly struct/member, method group conversion, null check over type check, top-level statements.
• Phase 3: Add resharper_place_field_attribute_on_same_line and resharper_wrap_object_and_collection_initializer_style to align JB cleanupcode output with dotnet format.
• Phase 4: Remove 14 .DotSettings inspection severities now covered by .editorconfig equivalents. Remove 2 duplicate mandatory imports already in Directory.Build.props.

Net result: .editorconfig reduced from 328 to 210 lines (-36%) with no loss of enforcement.

Verification

• dotnet build — 0 warnings, 0 errors
• dotnet format --verify-no-changes — passes
• dotnet build --configuration Release with EnforceCodeStyleInBuild=true — 0 warnings
• All tests pass (0 failures)

Test plan

• CI passes (build, format check, tests) on all matrix OSes
• Verify JB cleanupcode idempotency: dotnet format → jb cleanupcode --no-builtin-settings → git diff --exit-code

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