Include type definitions in the package

[gabmontes]

Types were added in #77 but not included in the NPM package, therefore type checking cannot be properly done. This is the error usually thrown by TypeScript:

Could not find a declaration file for module 'coinselect'. '.../node_modules/coinselect/index.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/coinselect` if it exists or add a new declaration (.d.ts) file containing `declare module 'coinselect';`

Including the type definitions file in the package as done in ChrisCho-H@f173732 and publishing a new version will solve the problem.

Note that I didn't bump the package version in this PR but happy to do so if that would help.

Possibly related to #85 too.

[Overtorment]

LGTM

[aaronisme]

any chance to publish an new version ?

[Overtorment]

@gabmontes can you pls pour fresh master into your branch? im hoping it will fix all failing CI jobs

[gabmontes]

@Overtorment done!

[Overtorment]

all passed, nice