Update dependency postcss to v8.4.39

This MR contains the following updates:

Package	Type	Update	Change
postcss (source)	devDependencies	patch	8.4.38 -> 8.4.39

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Release Notes

postcss/postcss (postcss)

v8.4.39

Compare Source

• Fixed CssSyntaxError types (by @​romainmenke).

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