Add gulp compile:dts task which generates single .d.ts from multiple files as workaround.

[mattmazzola]

This is a workaround for one of the biggest pain points in developing on the project which is manually updating the .d.ts file.

See: microsoft/TypeScript#4433

Steps:

1. Use tsd --declarations to generate a .d.ts file for each of the source files
2. Removes all the import statements (paths are not relevant since end goal is single file)
3. Concatenate all the .d.ts files to single powerbi.d.ts

Pros:
Autogenerates .d.ts

Cons:

• Can't use default exports since the concatenation would put multiple defaults in the same file
• Does not allow for encapsulation of internal interfaces due to concatenation.