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.

tests/unit/shared/Stubs/ScriptingDefinitionStub.ts

@@ -10,17 +10,17 @@ export class ScriptingDefinitionStub implements IScriptingDefinition {
 
   public endCode = 'REM end code';
 
-  public withStartCode(startCode: string): ScriptingDefinitionStub {
+  public withStartCode(startCode: string): this {
     this.startCode = startCode;
     return this;
   }
 
-  public withEndCode(endCode: string): ScriptingDefinitionStub {
+  public withEndCode(endCode: string): this {
     this.endCode = endCode;
     return this;
   }
 
-  public withLanguage(language: ScriptingLanguage): ScriptingDefinitionStub {
+  public withLanguage(language: ScriptingLanguage): this {
     this.language = language;
     return this;
   }