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.
Add support for expressions inside expressions.
Add support for templating where the output of one expression results in
another template part with expressions.
E.g., this did not work before, but compilation will now evaluate both
with expression with `$condition` and parameter substitution with
`$text`:
```
{{ with $condition }}
echo '{{ $text }}'
{{ end }}
```
Add also more sanity checks (validation logic) when compiling
expressions to reveal problems quickly.
