Do not construct dirname on Linux

[timdp]

I'm bundling term-size using esbuild, which handles import.meta.url a bit differently than other bundlers.

Now, in my case, the fileURLToPath call is dead code, because I'm running my bundle on Linux, where term-size doesn't need to invoke any commands. So I was thinking it could be more lazy, and coincidentally fix the issue I'm facing.

This PR moves the __dirname emulation into a helper function that's only called on macOS and Windows.

I've also put the line-splitting in there because I prefer DRY code, but it's a bit esoteric this way. Would you prefer an explicit splitLines (or even a dependency on split-lines)? Or alternatively, I could make a less invasive PR and only extract the __dirname construction into a helper function, for example.

... Or we could find a better way to deal with the lack of __dirname in ESM. 😉

[sindresorhus]

I usually don't like to change my perfectly valid code to please buggy/incomplete bundlers, but since this is a small change, I'll accept it.

[timdp]

I feel your pain. Thanks!