chore(deps): update dependency postcss to v8.4.19

This MR contains the following updates:

Package	Type	Update	Change
postcss (source)	devDependencies	patch	8.4.16 -> 8.4.19

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Release Notes

postcss/postcss

v8.4.19

Compare Source

• Fixed whitespace preserving after AST transformations (by Romain Menke).

v8.4.18

Compare Source

• Fixed an error on absolute: true with empty sourceContent (by Rene Haas).

v8.4.17

Compare Source

• Fixed Node.before() unexpected behavior (by Romain Menke).
• Added TOC to docs (by Mikhail Dedov).

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