Add HarGeneratorPlugin snippet, code lens, and documentation link

[garrytrinder]

Summary

Add support for the new HarGeneratorPlugin in the Dev Proxy Toolkit VS Code extension, following the recent addition in dotnet/dev-proxy#1421.

Tasks

• Add HarGeneratorPlugin to pluginSnippets and pluginDocs mappings in src/constants.ts
• Implement CodeLens support for HarGeneratorPlugin configuration sections in diagnostics
• Add documentation link for HarGeneratorPlugin in pluginDocs

References

• dotnet/dev-proxy#1421: Adds the HarGeneratorPlugin

Acceptance Criteria

• HarGeneratorPlugin snippet is available in pluginSnippets
• CodeLens appears for HarGeneratorPlugin configuration sections
• Documentation link for HarGeneratorPlugin is present in pluginDocs
• README and CHANGELOG updated if necessary

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Closes #1412 (see upstream PR)