[v5] Missing typings for 4 common sequelize data types

[billybarnum]

The tsc compiler is giving me error TS2339: Property 'xxx' does not exist for TINYINT, MEDIUMINT, SMALLINT, and NUMERIC data types, even though they still seem to be exported in lib/data-types.js in the underlying javascript repo.

I am using sequelize 5.0.0-beta.15 and @types/sequelize": "github:types/sequelize#c628e47acd0cb073b69d06b38ff3fd3ce3f47dbc" as dependencies, which I believe are the latest versions.