Refactor to enforce strictNullChecks

This commit applies `strictNullChecks` to the entire codebase to improve
maintainability and type safety. Key changes include:

- Remove some explicit null-checks where unnecessary.
- Add necessary null-checks.
- Refactor static factory functions for a more functional approach.
- Improve some test names and contexts for better debugging.
- Add unit tests for any additional logic introduced.
- Refactor `createPositionFromRegexFullMatch` to its own function as the
  logic is reused.
- Prefer `find` prefix on functions that may return `undefined` and
  `get` prefix for those that always return a value.

src/infrastructure/WindowVariables/WindowVariablesValidator.ts

@@ -17,7 +17,7 @@ export function validateWindowVariables(variables: Partial<WindowVariables>) {
 
 function* testEveryProperty(variables: Partial<WindowVariables>): Iterable<string> {
   const tests: {
-    [K in PropertyKeys<WindowVariables>]: boolean;
+    [K in PropertyKeys<Required<WindowVariables>>]: boolean;
   } = {
     os: testOperatingSystem(variables.os),
     isDesktop: testIsDesktop(variables.isDesktop),