create variable with glab code example

Example: glab variable

$ glab variable set WITH_ARG "some value"
$ glab variable set FROM_FLAG -v "some value"
$ glab variable set FROM_ENV_WITH_ARG "${ENV_VAR}"
$ glab variable set FROM_ENV_WITH_FLAG -v"${ENV_VAR}"
$ glab variable set FROM_FILE < secret.txt
$ cat file.txt | glab variable set SERVER_TOKEN
$ cat token.txt | glab variable set GROUP_TOKEN -g mygroup --scope=prod