Update dependency cssnano to v5.1.15

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
cssnano	5.1.7 -> 5.1.15

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Release Notes

cssnano/cssnano (cssnano)

v5.1.15: v5.1.15

Compare Source

Bug Fixes

• fix(postcsss-reduce-initial): fix mask-repeat conversion
• fix(postcss-colormin): don't minify colors in src declarations
• fix(postcss-merge-rules): do not merge conflicting flex and border properties

v5.1.14: v5.1.14

Compare Source

Bug Fixes

• fix: update autoprefixer and browserslist
• fix(postcss-reduce-initial): improve initial properties data

v5.1.13: v5.1.13

Compare Source

Bug Fixes

• fix TypeScript type of cssnano() return value (cssnano/cssnano@b92dbe3)

v5.1.12: v5.1.12

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Bug Fixes

• fix: preserve hyphenated property case (cssnano/cssnano@120a888)
• fix: ensure sorting properties does not break the output (cssnano/cssnano@0a3a133)
• fix: recognize 'constant' as a function (cssnano/cssnano@858a8b7)

v5.1.11: v5.1.11

Compare Source

Bug Fixes

• postcss-ordered-values: preserve constant values (32ab4d9b7d20d)

v5.1.10: v5.1.10

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Bug Fixes

• postcss-minify-selectors: preserve similar nested selectors (f6c29fb3e75ac26a2635d)

v5.1.9: v5.1.9

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Bug Fixes

• postcss-merge-longhand: preserve more color function fallbacks (712993cb4a4e0)

v5.1.8: v5.1.8

Compare Source

Bug Fixes

• postcss-convert-values: preserve percentage sign on IE 11 (57f060ad21734)

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