ShellManager: Change the env var detection regex to find more uses

The regex is now just '\bkak_\w+\b', no need for it to be looking
like a shell variable use.

This allows other tools that the shell to access Kakoune vars as
long as the name appears directly in the shell string.

src/shell_manager.cc

@@ -10,7 +10,7 @@
 namespace Kakoune
 {
 
-static const Regex env_var_regex(R"(\$\{kak_(\w+)[^}]*\}|\$kak_(\w+))");
+static const Regex env_var_regex(R"(\bkak_(\w+)\b)");
 
 ShellManager::ShellManager()
 {