Skip web builds in the plugin with typed target classification

[vishal-jadav]

Summary

• Skip Android transpilation and generated Android sources for SKIP_WEB builds.
• Centralize web/configuration/bridge/quiet/disabled environment flags in typed enums.
• Classify infrastructure targets and test harness sources with enums instead of raw string membership/comparison logic.
• Preserve existing Android and Apple build behavior when web mode is not enabled.

Validation

• swiftc -parse-as-library -typecheck Plugins/SkipPlugin/SkipPlugin.swift -module-name SkipPlugin -I /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/pm/PluginAPI -target arm64-apple-macosx13.0

This is the foundational companion change for the skip-ui, skip-web, and skipstone web-support PRs.

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