Don't use TypeScript const enum

[threepointone]

This PR replaces the const enum in index.d.ts with a regular enum. This lets boxen (or any packages that depend on it) be used in projects whose tsconfig has the setting isolatedModules:true.

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Hi! I was going to make an issue, but figured I'd make a PR with a suggested fix too.

Problem: I want to use update-notifier in a typescript project. My project has isolatedModules: true (because we're compiling with babel-preset-typescript, and trying to match variations across the two). Trying to typecheck this project fails with

$ node_modules/.bin/tsc
node_modules/boxen/index.d.ts:172:22 - error TS2748: Cannot access ambient const enums when the '--isolatedModules' flag is provided.

172  BorderStyle: typeof BorderStyle;
                         ~~~~~~~~~~~


Found 1 error.

which is expected, of course. But I can't seem to figure out a workaround for this that doesn't rely on using skipLibCheck: true, which would suck. Would it be possible for you to replace the const enum here with a regular enum? I made this change locally and it appeared to work fine, but I'm not clear what the broader implications are.

Please and thank you!