Create and publish a package with a shared LanguageVersion type

[parlough]

More and more Dart tools are relying on the Dart language version of a project or file, but there's no standard way to communicate about a language version. Some tools have used Version from package:pub_semver, but it's not meant for language versions and is a bit heavy weight. While others have created their own types to represent the information.

package:package_config does have a public LanguageVersion class, but it hasn't consistently been used. Perhaps because package developers think it's only for package config functionality or because it includes

A focused package with a LanguageVersion interface (or shared concrete implementation) would enable a small, stable dependency that different tools and packages can use to share Dart language versions. Related types, such as Version in package:pub_semver could add extension methods to convert both directions as well.

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A few users of language version and their solution.

• package:package_config - Custom LanguageVersion class in public API
• dart-lang/build - The LanguageVersion type from package:package_config (Example usage)
• pkg/_macros - Custom LanguageVersion interface
• dart-lang/pub - Custom LanguageVersion class with support for comparisons
• dart-lang/dart_style - Version from package:pub_semver
• analyzer - Version from package:pub_semver