Add TypeScript type declarations

[craiganderson-iotv]

Hi!

Firstly, thanks for your work on this project!

I used patch-package to patch merge-package.json@7.0.1 for the project I'm working on.

Unfortunately, this package has no suitable type declarations, and so cannot be natively used within a TypeScript project as it lacks a suitable .d.ts file. I've autogenerated one using tsc:

diff --git a/node_modules/merge-package.json/src/index.d.ts b/node_modules/merge-package.json/src/index.d.ts
new file mode 100644
index 0000000..e55246a
--- /dev/null
+++ b/node_modules/merge-package.json/src/index.d.ts
@@ -0,0 +1,2 @@
+declare function _exports(_currentPackageJson: string, _fromPackageJson: string, _toPackageJson: string): string;
+export = _exports;

It would be very helpful if this (or more proper) type definitions could be added for everyone to use.

This issue body was partially generated by patch-package.