feat: initial parser for todo.txt format with spec and test cases

- Add parser for todo.txt tasks supporting priority, dates, projects,
  contexts, and key:value metadata
- Include full todo.txt format specification (SPECS.md) and visual
  description (description.svg)
- Add sample test.todo.txt file with valid, borderline, and malformed
  cases
- Initialize Go module and main entrypoint for parsing and JSON output
- Add .gitignore for binary artifacts

.gitignore

@@ -0,0 +1 @@
+todotxt2remind