Add env variable to disable transpiling with typescript

[OzzieOrca]

• Allows developer to force TS to be compiled with babel

I'm having trouble using tsc to compile dangerfile.ts and it's currently throwing SyntaxError: Cannot use import statement outside a module.

My workflow runs all .ts files through Babel only, uses tsc as a linter only, and I have target set to esnext in my tsconfig.json file:

{
    "compilerOptions": {
        "target": "esnext",
        ...
    },
}

Danger seems to be transpiling with tsc but node can't execute its output since the target is so high. This PR adds the DANGER_DISABLE_TSC flag for a developer to skip tsc and use Babel.

Idk if this is desired to be added to this project and I'm open to other implementations. Maybe the logic should be less about disabling tsc and more about forcing Babel. Or maybe a more robust solution to specify a specific transpiler would be better such as DANGER_TRANSPILATION=off|babel|tsc.

[orta]

Yeah, I think this is fine - I doubt there'll ever be a third transpiler option, so in theory with these two options you can set it to any of the three.

[orta]

Shipped in 10.6.0