feat(parsers): add Dart / Flutter language support

[tob-scott-a]

Uses tree-sitter-language-pack's dart grammar (transitive dep, no new installs). Extracts:

• Top-level functions (paired from function_signature + function_body sibling nodes; Dart's grammar keeps them as separate AST siblings rather than nesting them, which required a custom signature-pairing walker).
• Classes, including abstract classes.
• Methods inside class bodies (method_signature + function_body pairs).
• Abstract methods (declaration -> function_signature, no body).
• Parameters and return types.
• Imports — package: and dart: URIs captured as dependencies.

Calls in Dart aren't a single AST node — they're an identifier followed by a selector carrying an argument_part. Added _collect_dart_calls to walk bodies, detect argument-bearing selectors, and reconstruct the callee name from the preceding identifier/selector chain (handles both plain calls b() and method calls obj.method()).

Entrypoint detector:

• @pragma('vm:entry-point') — Dart's explicit native-callable marker. Used for FFI callbacks, platform-channel handlers, and deferred loading targets; these are attacker-reachable from the host platform.

Flutter lifecycle methods (build, initState, dispose) are NOT flagged by default because they run in-process and aren't directly attacker-reachable. Users who want them tagged can add them via the override file.

Known gaps (documented in the parser docstring): throw statements aren't yet added to exception_types — parallel to Swift/Kotlin, the throw AST node overlaps with other control-transfer forms.

12 new tests: Dart parser, Dart entrypoint detection. README supported-languages and framework-coverage tables updated.

Roadmap left before v0.2.0: Option A (Go/Ruby/C++ entrypoint detectors) and Option B (branches + docstring in JSON export).