duration / time literals

[bcho]

for ago:

querying select * from ku_cli_26GgJgKcmoovUpimJHKWyY_raw where ts > ago(3h) limit 100
react_devtools_backend.js:4026 
        
       RpcError: query failed: unrecognized token: "3h"
    at grpc-web-transport.js:123:23

[bcho]

ref: https://www.sqlite.org/lang_datefunc.html