feat(functions): auto-detect native build hook support for dart functions (#10958)

* wip: auto-detect native assets support via package_config.json language version

* docs(functions): trim unprecedented comments in dart native-assets detection

* fix(functions): handle non-object package_config.json in dart feature detection

* refactor(functions): move dart sdk version thresholds into DartVersionFeatures

* refactor(functions): rename DART_LEGACY_EXECUTABLE_PATH and dedupe its usage

* Adding CHangelog

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Co-authored-by: Joe Hanley <joehanley@google.com>