BREAKING(dotenv): remove examplePath option from load[Sync]()

[iuioiua]

What's changed

The examplePath option for load[Sync]() from @std/dotenv has been removed. The allowEmptyValues option and MissingEnvVarsError have also been removed, as the basis for their existence was dependent on the examplePath option.

Motivation

See details in #5449.

Migration guide

To migrate, if environment variables need to be validated, do so in-code instead of using the examplePath option.

# .env.example
# Notice: missing `QUX` environment variable
FOO=bar

import "@std/dotenv/load";

+ if (!Deno.env.has("QUX") {
+   // ...
+ }

Related

Closes #5449

[kt3k]

.env.example is a common file name for showing the examples of what should be in .env file. https://github.com/search?q=path%3A.env.example&type=code

However using that file as the validator of .env file feels like an opinionated interpretation of it, and doesn't seem fitting to the policy of std. This removal seems reasonable to me.

[iuioiua]

I'll merge in a few days if no strong objections arise.