feat: introduce findMatchingDecl? for code quality checks in Lean.Linter

[wkrozowski]

This PR adds Lean.Linter.findMatchingDecl?, which finds the declaration a piece of syntax belongs to by searching the current command's info trees for declarations whose declaration range contains it, together with Lean.Linter.findCodeQualitySource? and Lean.Linter.findCodeQualitySource, which turn that declaration into a code-quality Source (the latter falling back to the current module when no declaration matches). This gives linters a way to attribute diagnostics and code-quality entries to the declaration being elaborated.

The helpers are exercised by a new package test, tests/pkg/find_matching_decl, which registers a linter reporting the matched declaration and derived source for each command. It covers plain definitions, inductives, mutual blocks, classes, instances, and commands with no associated declaration.

[mathlib-lean-pr-testing]

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[leanprover-bot]

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