fix(build): fix extends in ts-config.json

[alan-agius4]

Short description of what this resolves:

While typescript support extends of another tsconfig, a limitation in validateRequiredFilesExist method doesn't as it's reading the file using fs, rather than getTsConfig which is a wrapper of the TypeScript API,

Changes proposed in this pull request:

• Impalement getTsConfigAsync method
• In validateRequiredFilesExist use it rather than readFileAsync for the tsconfig.json

Note: I cannot run Unit Tests

Fixes: #745

[danbucholtz]

@alan-agius4,

Thanks!

Thanks,
Dan