Factor out lookupExecutable and other PATH improvments

This ended up motivating a good deal of other infra improvements in
order to get Windows right:

- `OsString` to complement `std::filesystem::path`

- env var code for working with the underlying `OsString`s

- Rename `PATHNG_LITERAL` to `OS_STR`

- `NativePathTrait` renamed to `OsPathTrait`, given a character template
  parameter until #9205 is complete.

Split `tests.cc` matching split of `util.{cc,hh}` last year.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libutil/windows/environment-variables.cc

@@ -4,7 +4,30 @@
 
 namespace nix {
 
-int unsetenv(const char *name)
+std::optional<OsString> getEnvOs(const OsString & key)
+{
+    // Determine the required buffer size for the environment variable value
+    DWORD bufferSize = GetEnvironmentVariableW(key.c_str(), nullptr, 0);
+    if (bufferSize == 0) {
+        return std::nullopt;
+    }
+
+    // Allocate a buffer to hold the environment variable value
+    std::wstring value{L'\0', bufferSize};
+
+    // Retrieve the environment variable value
+    DWORD resultSize = GetEnvironmentVariableW(key.c_str(), &value[0], bufferSize);
+    if (resultSize == 0) {
+        return std::nullopt;
+    }
+
+    // Resize the string to remove the extra null characters
+    value.resize(resultSize);
+
+    return value;
+}
+
+int unsetenv(const char * name)
 {
     return -SetEnvironmentVariableA(name, nullptr);
 }
@@ -14,4 +37,9 @@ int setEnv(const char * name, const char * value)
     return -SetEnvironmentVariableA(name, value);
 }
 
+int setEnvOs(const OsString & name, const OsString & value)
+{
+    return -SetEnvironmentVariableW(name.c_str(), value.c_str());
+}
+
 }