Add get_location function to retrieve AST node position

[harehare]

[rrajpuro]

+1 on this. Additional use case: when using -F json output, having line_start, line_end, and column fields on every node would enable:

1. Editor integration — jump to matched nodes in VS Code / Neovim
2. Combining mq results with line-based tools (sed, awk)
3. CI/CD linting — report "problem at line X" for malformed markdown
4. A simpler --with-location or -n flag for non-JSON output (like grep's -n)

Related: #1625 (closed as dup of this)