fix(create-twilio-function): adds rootDir to generated tsconfig.json

[philnash]

By default, tsc deems the lowest common directory as the root directory
of the application and delivers the compiled files into the outDir.
If you do not have an assets directory with a .ts file in it, then tsc
deems src/functions as the root directory and puts the compiled
functions directly into the dist directory. This wasn't obvious,
because the example generated files included a private TS asset.

Setting the rootDir to "src" ensures that the base directory is "src"
and a "functions" directory is generated in dist.

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