brew-mas: add page (#1898)

pages/osx/brew-mas.md

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+# brew mas
+
+> Mas is a simple command line interface for the Mac App Store.
+
+- Search the Mac App Store by app name and return matching identifiers:
+
+`mas search {{app_name}}`
+
+- Install or update a previously purchased application:
+
+`mas install {{app_name}} {{app_identifier}}`
+
+- Show all installed applications and their product identifiers:
+
+`mas list`
+
+- List installed apps with pending updates:
+
+`mas outdated`
+
+- Install all pending updates:
+
+`mas upgrade`
+
+- Install updates for a specific app:
+
+`mas upgrade {{app_identifier}}`