Replace manual whitespace loops in is_exit_meta_command with strspn

[python-processing-unit]

Problem

src/main.c:133-135,140-142 uses manual while loops to skip whitespace characters. strspn(p, " \t\r\n") does this in one call.

Fix

Replace both loops with p += strspn(p, " \t\r\n");.

Reported by Kilo Code (automated audit tool), not a human reviewer.