Measure nesting depth

A style checker for indentation-based config files reports how deeply a file nests. Each non-blank line is indented by exactly 2 spaces per nesting level (top-level lines have no indent and count as depth 1, a line indented 2 spaces is depth 2, and so on). Whitespace-only lines are skipped. Given the file as a list of lines, return the maximum depth of any non-blank line, or 0 for a file with no non-blank lines. The indentation is guaranteed consistent, your job is only to measure it.