Continuation of #1002, add _position to deprecation whitelist

[caseywebdev]

What does this PR do?

This PR prevents the internal position function from throwing deprecation warnings because of the use of unpack and is-length.

If this is related to an existing issue, include a link to it as well.

#1001, #1002

Screenshots (if relevant)

When using @include positon(...) in v4.3.1...

WARNING: [Bourbon] [Deprecation] `unpack` is deprecated and will be removed in 5.0.0.
Backtrace:
	node_modules/bourbon/app/assets/stylesheets/functions/_unpack.scss:19, in function `unpack`
	node_modules/bourbon/app/assets/stylesheets/addons/_position.scss:32, in mixin `position`
	stdin:4

WARNING: [Bourbon] [Deprecation] `is-length` is deprecated and will be removed in 5.0.0.
Backtrace:
	node_modules/bourbon/app/assets/stylesheets/functions/_is-length.scss:9, in function `is-length`
	node_modules/bourbon/app/assets/stylesheets/addons/_position.scss:44, in mixin `position`
	stdin:4

[tysongach]

Thanks @caseywebdev!

Merged as 5e6f787.

[tysongach]

We just released v4.3.2 which includes these changes.

[caseywebdev]

Awesome, thanks!