Add exists command

HELP.md

@@ -446,6 +446,12 @@ Assuming the current subject is a valid single data document, convert it `from`
 Supports: `php`|`json`
 Example: `{"a": 1}` -> `convert json php` -> `['a' => 1]`
 
+#### `exists <fs|var> <name>`
+Checks whether the given file or variable exists.
+Supports: `fs` for filesystem checks, `var` for variable names
+Example: `$a = 1` -> `exists var a` -> `1`
+Example: `exists fs fake-file.txt` -> `0`
+
 #### User-specific setup files
 The file `~/.str.rc` is automatically executed when `str` starts up.
 You can use this to define a user-specific environment.