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/NodeStateChangeEventArgsStub.ts

@@ -23,7 +23,7 @@ export class NodeStateChangeEventArgsStub implements NodeStateChangeEventArgs {
     return this;
   }
 
-  public withOldState(oldState: TreeNodeStateDescriptor): this {
+  public withOldState(oldState: TreeNodeStateDescriptor | undefined): this {
     this.oldState = oldState;
     return this;
   }