docs: overhaul Copilot instructions with coding conventions and prompt files (#40113)

* docs: overhaul Copilot instructions with coding conventions and prompt files

Major update to .github/copilot-instructions.md:
- Add coding conventions (naming, error handling, RAII, strings, headers,
  synchronization, localization, telemetry, formatting, IDL/COM, config)
- Add test authoring summary pointing to detailed test.md prompt
- Add namespace-to-directory map for top-level namespaces
- Add key source files list (defs.h, WslTelemetry.h, wslc.idl, etc.)
- Replace clang-format references with .\FormatSource.ps1
- Consolidate duplicate timing info into single reference table

New files:
- .github/copilot/review.md: Review prompt focused on high-risk areas
  (ABI breaks, missing localization, resource safety)
- .github/copilot/test.md: Test generation prompt with TAEF patterns
- .github/copilot/commit.md: Commit message guidelines
- .editorconfig: Editor settings for non-C++ files

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* docs: address PR review feedback

- Scope precomp.h guidance to Windows components (Linux doesn't use it)
- Fix review.md reference to .github/copilot-instructions.md
- Restore clang-format as Linux formatting option alongside FormatSource.ps1
- Note FormatSource.ps1 requires cmake . first
- Fix en-us -> en-US casing

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.editorconfig

@@ -0,0 +1,43 @@
+# EditorConfig — https://editorconfig.org
+# Complements .clang-format for non-C++ files
+
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = crlf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.{cpp,h,hpp,c,idl}]
+# C/C++ formatting is handled by .clang-format
+indent_size = 4
+
+[*.{yml,yaml}]
+indent_size = 2
+
+[*.{json,jsonc}]
+indent_size = 2
+
+[*.{md,markdown}]
+trim_trailing_whitespace = false
+
+[*.py]
+indent_size = 4
+
+[*.{ps1,psm1,psd1}]
+indent_size = 4
+
+[*.{xml,resw,resx,props,targets,nuspec}]
+indent_size = 2
+
+[*.cmake]
+indent_size = 4
+
+[CMakeLists.txt]
+indent_size = 4
+
+[Makefile]
+indent_style = tab