add flow analyser for diagnostics

[kartikangiras]

Description

• Uses a pure-JavaScript analyzer that employs regex and indentation heuristics to parse Python source code without needing a full AST or Python runtime.
• VS Code Integration: Maps analysis results to a DiagnosticCollection, surfacing errors (squiggly underlines) in real-time as the user types, opens, or saves a file.
  -Validates the presence of required start and end steps within FlowSpec classes.
• Detects unreachable steps (not connected to start) and identifies undefined step references in self.next() calls.
• Flags non-end steps that fail to transition to another step via self.next.

Issue

fixes #7

Working Demo:

Screen.Recording.2026-03-07.at.01.38.45.mov