Implement elements-match checker

[mmorel-35]

Adds the elements-match checker from the CONTRIBUTING.md open-for-contribution list, which detects verbose slice comparison patterns that should use assert.ElementsMatch.

What it detects

// ❌ Verbose: sort both slices, then compare
slices.Sort(expected)
slices.Sort(result)
assert.True(t, slices.Equal(expected, result))

// ✅ Use the testify idiom
assert.ElementsMatch(t, expected, result)

Changes

• internal/checkers/elements_match.go — AdvancedChecker that walks ast.BlockStmt nodes looking for the 3-statement triplet slices.Sort(x) + slices.Sort(y) + assert.True(t, slices.Equal(x, y)). Handles commuted sort order and both slices (Go 1.21) and golang.org/x/exp/slices. Includes a full autofix that removes the two sort statements and rewrites the assertion.
• internal/testgen/gen_elements_match.go — Test generator with errored and golden templates.
• Registry — Registered as disabled by default (matching the "maybe" stability note in CONTRIBUTING.md).
• README.md — Added to the checkers table and reference section.
• CONTRIBUTING.md — Removed from the open-for-contribution list.

[ccoVeille]

I had no idea ElementsMatch existed 😱😅🤣