Enable introspecting pulley at runtime

[alexcrichton]

This commit includes a few assorted changes to improve detection of whether Wasmtime is using Pulley at runtime:

• A new Engine::is_pulley API is added.
• A new wasmtime_engine_is_pulley C API is added.
• The Config::target method is now available regardless of compilation features.
• The Engine::target method now consults the Config to have the same fallback logic for when a target is not explicitly configured.

cc #1980 (comment)

[github-actions]

Label Messager: wasmtime:config

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