Add module to types

[AlexBroadbent]

Fixed #8

Resolves the error when building:

yarn run v1.22.11
$ tsc
node_modules/string-sanitizer/index.d.ts:13:1 - error TS1046: Top-level declarations in .d.ts files must start with either a 'declare' or 'export' modifier.

13 const sanitize: ISanitize;
   ~~~~~


Found 1 error.

error Command failed with exit code 2.