StandardSql: Expose Macros

[shelldandy]

Seems like the macros used on standardSql match the ones being used by sqlds, it would be good to expose them so other sql datasources that use sqlds can use them to build on top of instead of copy/pasting them

https://github.com/grafana/grafana-experimental/blob/main/src/sql-editor/standardSql/macros.ts