Fix placeholder-pdf-lib's .d.ts

[vbuch]

Fix the reported missing import of pdf-lib's PDFPage that we use as a parameter type.
To automate the process I additionally introduce tsd as a tool to validate the types we generate. Only doing an import with no additional type tests.

Fixes #234

[coveralls]

coverage: 99.636%. remained the same
when pulling 7749d8d on bugfix/234-pdf-lib-dts
into 13b071e on develop.