`termux-api` command should detect if Termux:API plugin is not installed and print error.

Feature description
You can invoke command like echo $(pm path com.termux.api 2>&1 </dev/null) in termux to get termux:api plugin apk path or get error otherwise, it should be usable to detect plugin installation even without PackageManager API (or JVM).

Background information
Termux:X11 companion package can detect if main APK is installed or not via PackageManager API. Termux:API should do that too.
There are enough people coming to discord, matrix and reddit complaining about termux-api-based commands are hanging indefinitely.

Redirecting everything to /dev/null or anonymous pipes is necessary in command echo $(pm path com.termux.api 2>&1 </dev/null), the same applies to the case if you are porting it to C.

Related to:
https://github.com/termux/termux-api-package/issues/138
https://github.com/termux/termux-api-package/issues/153
https://github.com/termux/termux-api-package/issues/80

The issue names the termux-api command and the pm path com.termux.api check; start by locating that command's implementation and reviewing how it invokes the plugin. Done means the command reports a clear error when the plugin is absent, succeeds when installed, and does not hang; exercise both cases while suppressing command output as described.