fix: crash when devDependencies undefined using ts-standard

[Flet]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[✅ ] Bug fix
[ ] New feature
[ ] Other, please explain:

What changes did you make? (Give an overview)

Problem

• If a package.json file does not have a devDependencies property, the plugin explodes.
• Example output when things go bad:

[Info  - 7:52:50 PM] JavaScript Standard Style server stopped.
[Info  - 7:52:51 PM] JavaScript Standard Style server is running.
(node:7171) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'ts-standard' of undefined
   at /Users/flet/.vscode/extensions/standard.vscode-standard-2.0.0/server/out/server.js:179:84
(Use `Code Helper (Renderer) --trace-warnings ...` to show where the warning was created)
(node:7171) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
(node:7171) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
(node:7171) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'ts-standard' of undefined
   at /Users/flet/.vscode/extensions/standard.vscode-standard-2.0.0/server/out/server.js:179:84
(node:7171) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 3)

Fix

• Added optional chaining operator ?. to avoid exploding when checking for ts-standard in devDependencies

Which issue (if any) does this pull request address?

• I did not see any related open issues.

Is there anything you'd like reviewers to focus on?

• No

[welcome]

🙌 Thanks for opening this pull request! You're awesome.

[welcome]

🎉 Congrats on getting your first pull request landed!

[theoludwig]

Thanks for your PR! @Flet 🚀

Could you review my PR for releasing 2.0.1 so we can release a new version? 😄
See: #311