case: add page (#1930)

pages/common/case.md

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+# case
+
+> Branch based on the value of an expression.
+
+- Match a variable against string literals to decide which command to run:
+
+`case {{$tocount}} in {{words}}) {{wc -w README}}; ;; {{lines}}) {{wc -l README}}; ;; esac`
+
+- Combine patterns with |, use * as a fallback pattern:
+
+`case {{$tocount}} in {{[wW]|words}}) {{wc -w README}}; ;; {{[lL]|lines}}) {{wc -l README}}; ;; *) {{echo "what?"}}; ;; esac`