Add warnings for 'exit' similar to 'return' (fixes #1388)

CHANGELOG.md

@@ -2,14 +2,17 @@
 ### Added
 - Command line option --severity/-S for filtering by minimum severity
 - Command line option --wiki-link-count/-W for showing wiki links
+- SC2152/SC2151: Warn about bad `exit` values like `1234` and `"foo"`
 - SC2236/SC2237: Suggest -n/-z instead of ! -z/-n
 - SC2238: Warn when redirecting to a known command name, e.g. ls > rm
 - SC2239: Warn if the shebang is not an absolute path, e.g. #!bin/sh
 - SC2240: Warn when passing additional arguments to dot (.) in sh/dash
 - SC1133: Better diagnostics when starting a line with |/||/&&
+
 ### Changed
 - Most warnings now have useful end positions
 - SC1117 about unknown double-quoted escape sequences has been retired
+
 ### Fixed
 - SC2021 no longer triggers for equivalence classes like '[=e=]'
 - SC2221/SC2222 no longer mistriggers on fall-through case branches