Feat: Allow typescript handler

[piaverous]

This PR addresses the following issue: #85

It allows the user to set a global config parameter tsHandler: true, to generate a warmup function written in Typescript instead of Javascript

As mentioned in the issue, serverless/serverless-plugin-typescript#79 should have helped fixing this issue. However, in case of a repo whose code is fully typescript, and where tsconfig.json contains allowJs: false, you would still get the following error

{ file: undefined,
  start: undefined,
  length: undefined,
  messageText: 'File \'_warmup/index.js\' has unsupported extension. The only supported extensions are \'.ts\', \'.tsx\', \'.d.ts\'.',
  category: 1,
  code: 6054,
  reportsUnnecessary: undefined }

It was therefore not enough, and having the handler written in JS still caused an issue.

[juanjoDiaz]

HI @piaverous ,

Thanks for the contribution.
I'm still pretty certain that we shouldn't support more than one handler.
I can't find any mention to allowJs in the issue that you reference.
In any case, it seems clear to me that the right behaviour of the typescript plugin would be to just ignore JS handlers (so the TS settings should be irrelevant). So I'd still say that the problem is on the serverless-plugin-typescript.

Wdyt?

[juanjoDiaz]

Closing for the reasons stated above.

[ghost]

The suggested code work for me.