feat: add is_mapping typing helper

[msto]

Summary

This PR is the fourth in a stack to expand Metric's collection parsing behavior to support tuple, set, and dict along with the existing list support, to provide parity with fgpyo.

This PR simply adds a small is_mapping helper that identified dict[K, V] types. It's added in its own PR to make the ensuing PR (which adds DelimitedMapping) easier to review.

Related Stack

• refactor: rename DelimitedList to DelimitedCollection #80
• feat: extend DelimitedCollection to set, frozenset, and tuple #81
• refactor: extract _DelimitedConverter base from DelimitedCollection #82
• feat: add is_mapping typing helper #83
• feat: add DelimitedMapping converter for dict fields #84

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

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Run ID: a7fdc633-06ef-4e34-a750-5e78fefda538

📥 Commits

Reviewing files that changed from the base of the PR and between 2615c70 and 1e12346.

📒 Files selected for processing (2)

• fgmetric/_typing_extensions.py
• tests/test_typing_extensions.py

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