Add backdrop-filter to properties list

[controversial]

What do the changes you have made achieve?
Added backdrop-filter as a known property. This is used fairly widely, most notably on some pages of Apple's website. It's available (with -webkit-) on safari (desktop and mobile), and is available without prefix in recent Chrome Canary builds.

Although it's only widely available with prefix, it still shows as a misspelled property in configs that have vendor prefix warnings disabled.

Are there any new warning messages?
No
Have you written tests?
Nope. If this is the kind of thing that needs tests, let me know, and I'll gladly write one
Have you included relevant documentation
Is documentation relevant for this?
Which issues does this resolve?
No open issues

<DCO 1.1 Signed-off-by: Luke Deen Taylor luke@deentaylor.com>

[coveralls]

Coverage remained the same at 97.374% when pulling 22e7fef on controversial:patch-1 into a4db3f8 on sasstools:develop.

[coveralls]

Coverage remained the same at 97.374% when pulling 22e7fef on controversial:patch-1 into a4db3f8 on sasstools:develop.

[DanPurdy]

@controversial I'm not sure about this, it's currently listed as a non official property and under an editors draft, with usage only possible with a vendor prefix. Can you not just add it to the extra properties option within the rule instead?

https://github.com/sasstools/sass-lint/blob/develop/docs/rules/no-misspelled-properties.md

@sasstools/sass-lint-contributors any opinions?

[controversial]

Yeah, I guess I could. While it's not the end of the world if this doesn't get merged, what's the argument for not merging it? The way I see it,

• It's not similar enough to any other properties that it could realistically cause false positives with spelling
• It's already available in Chrome without prefix, it's just hidden behind a flag, which seems to indicate that it's on the path to being supported.
• It's been used in production by Apple and a few others for more than a year.

So... why not merge it? It seems to me like there are only advantages, no disadvantages.