Predefined Variable Support

[blueray453]

Please add support for predefined variables like ${file}, ${workspaceFolder}, ${selectedText} etc. mentioned in https://code.visualstudio.com/docs/editor/variables-reference#_predefined-variables so that we can use commands like:

sd before after ${file}
sd before after ${selectedText}
sed -i 's/old-text/new-text/g' ${file}
sed -i 's/old-text/new-text/g' ${selectedText}
${selectedText} | tr "a-z" "A-Z"
cat ${file} | tr "a-z" "A-Z"

These command examples are just to give a rough idea.