feat: add detection for 15 new Dependabot package ecosystems

[zkoppert]

Summary

Adds file-based detection for 15 new Dependabot-supported package ecosystems, expanding coverage from 14 to 29 ecosystems.

New ecosystems

Ecosystem	YAML value	Manifest files
Bazel	bazel	MODULE.bazel, WORKSPACE, WORKSPACE.bazel
Bun	bun	bun.lock
Conda	conda	environment.yml, conda.yaml
Docker Compose	docker-compose	docker-compose.yml, compose.yaml
.NET SDK	dotnet-sdk	global.json
Elm	elm	elm.json
Git submodule	gitsubmodule	.gitmodules
Helm	helm	Chart.yaml
Julia	julia	Project.toml
pre-commit	pre-commit	.pre-commit-config.yaml
Pub (Dart)	pub	pubspec.yaml
Rust toolchain	rust-toolchain	rust-toolchain.toml
Swift	swift	Package.swift
uv	uv	uv.lock
vcpkg	vcpkg	vcpkg.json

Files changed

• dependabot_file.py: Added new ecosystems to package_managers_found and package_managers dicts
• env.py: Added all 15 ecosystems to SUPPORTED_PACKAGE_ECOSYSTEMS (now 29 total)
• test_dependabot_file.py: Added parameterized subTest covering all 15 new ecosystems
• README.md: Updated EXEMPT_ECOSYSTEMS docs to list all 29 supported ecosystems

Not included

OpenTofu is deferred - it shares .tf files with terraform, so detection overlap needs separate design consideration.

Closes #515

Testing

• All 158 tests + 15 subtests pass
• 99% code coverage (above 80% threshold)
• make lint passes clean (pylint 10/10)