Post-merge-review: Fix template-no-multiple-empty-lines: detect trailing empty lines and fix reported location

[johanrd]

What's broken on master

Two bugs in the line-scanning loop:

1. Trailing empty lines silently ignored. The rule reports only when a non-empty line is encountered after a streak of empty lines, so a file ending with …foo\n\n\n (three newlines) never reports. Upstream handles this by iterating allLines.length + 1 (no-multiple-empty-lines.js L49).
2. Inverted error location. Master computes start.line = firstEmptyLine + max + 2 and end.line = index, producing start.line > end.line in real cases.

Fix

• Swallow the final newline — strip the last element when it's empty, matching upstream L15 (source.at(-1) === '' ? source.slice(0, -1) : source).
• Add post-loop reportExcess call for trailing empty lines.
• Correct the loc computation.
• Extract the reporting into a shared reportExcess function so both the mid-file and trailing cases use identical logic.

Test plan

• 27/27 tests pass on the branch
• 2 new trailing-empty-line tests fail on master (0 errors reported, 1 expected)
• 1 new test with explicit line/endLine assertions fails on master (the inverted loc bug)

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Co-written by Claude.