Add .ts to resolve.extensions

[mottox2]

@sw-yx Please check this.

Fix #79. I add '.ts' to resolve.extensions, so we can use these extensions as import.

refs. https://webpack.js.org/configuration/resolve/#resolve-extensions

Test

in Application directory with lambda

$ yarn add -D @babel-preset-typescript
$ ../netlify-lambda/bin/cmd.js build src/

netlify.toml

[build]
  functions = "functions"
  publish = "public"
  command = "npm run build"

.babelrc

{
  "presets": [
    "@babel/preset-typescript",
    [
      "@babel/preset-env",
      {
        "targets": {
          "node": "6.10.3"
        }
      }
    ]
  ],
  "plugins": [
    "@babel/plugin-proposal-class-properties",
    "@babel/plugin-transform-object-assign",
    "@babel/plugin-proposal-object-rest-spread"
  ]
}