Add .env.sample as Dotenv filename

[spenserblack]

This adds the .env.sample filename for the Dotenv language

Checklist:

• I am adding a new extension filename to a language.
  • The new extension filename is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A%2F%28%5E%7C%5C%2F%29%5C.env%5C.sample%24%2F
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/diesel-rs/diesel/blob/9d8d1ee7ed8ea1ad8759492fb727747c8b34055f/.env.sample
    • Sample license(s):
      • Apache 2.0
      • MIT
      • (License is Apache 2.0 or MIT according to the project's Cargo.toml)
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.