chore: migrate from .sln to .slnx with reorganized folder structure (#4984)

Replace TUnit.sln with the new TUnit.slnx XML format via
`dotnet sln migrate`, then reorganize all 86 projects into logical
folders for a better IDE experience:

- /Core/ — TUnit, TUnit.Core, TUnit.Engine, TUnit.Assertions
- /Libraries/ — Aspire, AspNetCore, FsCheck, Playwright, Logging, Mocks
- /Analyzers/ — all Roslyn analyzers and code fixers
- /SourceGenerators/ — all source generators and Roslyn variants
- /Tests/ — all test projects
- /Benchmarks/ — source generator benchmarks
- /Build/ — TUnit.Pipeline
- /Examples/ — playground and example projects
- /Templates/ — project templates (unchanged)

No projects were at the solution root previously without a folder;
now zero remain orphaned. Update .vscode/tasks.json and docs
Dockerfile example references.

Author: thomhurst

.vscode/tasks.json

@@ -7,7 +7,7 @@
             "type": "process",
             "args": [
                 "build",
-                "${workspaceFolder}/TUnit.sln",
+                "${workspaceFolder}/TUnit.slnx",
                 "/property:GenerateFullPaths=true",
                 "/consoleloggerparameters:NoSummary;ForceNoAlign"
             ],
@@ -19,7 +19,7 @@
             "type": "process",
             "args": [
                 "publish",
-                "${workspaceFolder}/TUnit.sln",
+                "${workspaceFolder}/TUnit.slnx",
                 "/property:GenerateFullPaths=true",
                 "/consoleloggerparameters:NoSummary;ForceNoAlign"
             ],
@@ -33,7 +33,7 @@
                 "watch",
                 "run",
                 "--project",
-                "${workspaceFolder}/TUnit.sln"
+                "${workspaceFolder}/TUnit.slnx"
             ],
             "problemMatcher": "$msCompile"
         }