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This allows go-to-definition to reach the original .ts files for
consumers of the package. It emits .d.ts.map files alongside the regular
.d.ts files

e.g. in sdk.createClient go-to-definition on createClient will place the
cursor at the function in src/matrix.ts rather than inside a definition
file lib/matrix.d.ts
Signed-off-by: Alex Macleod <alex@macleod.io>