Add entry for Vale rule: allow.yml

[daobrien]

When we create rules in Vale we need to make sure that they're based on solid
guidance, and that we can refer writers to this guidance for more info.

allow.yml tests for "allows" and "permits" and suggests using "enables" or
similar instead. Other options are available of course, such as supports,
provides, lets, facilitates, etc. This is too much info for the Vale message,
but we should include it in the style guide.

Ideally, we should also ensure that these entries have IDs so that we can link
directly to them in Vale rules and also in editor snippet entries.

[julian-cable]

This item is covered in the "Anthropomorphism" section:
https://stylepedia.net/style/5.1/#Avoiding_Slang_Metaphors-MisleadingLanguage-Anthropomorphism

Rather than replacing "allow" or "permit" with a different verb, I usually find that a better approach is to rewrite the sentence to make the user the subject. So I would not see this one as a case of replacing verb 'a' with verb 'b'.

[daobrien]

The user is not necessarily the subject. If you set 664 perms on a file then those perms allow the user and group to write to that file. The permissions are doing the allowing, not the user. The "user" might not be a person, either. It might be a service or some other agent. There are many examples.

[julian-cable]

Addressed in #643.