Add GitHub Security Advisory data for Dart packages

[westonsteimel]

What would you like to be added:
Add support for pulling in GitHub Security Advisory data for Dart

Why is this needed:
Syft supports cataloging of Dart packages but there is currently no vuln data being built into grype-db to match against in grype

Additional context:
Should hopefully just be a simple add to the mapping in:

vunnel/src/vunnel/providers/github/parser.py

Lines 32 to 41 in cd2f18e

	ecosystem_map = {
	"COMPOSER": "composer",
	"GO": "go",
	"MAVEN": "java",
	"NPM": "npm",
	"NUGET": "nuget",
	"PIP": "python",
	"RUBYGEMS": "gem",
	"RUST": "rust",
	}