font-source-sans-pro: add a variable variant

packages/font-source-sans-pro/CHANGELOG.md

packages/font-source-sans-pro/build.sh

...sans-pro/files/source-sans-variable-base-ext-italic.woff2

...sans-pro/files/source-sans-variable-base-ext-normal.woff2

...rce-sans-pro/files/source-sans-variable-base-italic.woff2

...rce-sans-pro/files/source-sans-variable-base-normal.woff2

...rce-sans-pro/files/source-sans-variable-rest-italic.woff2

...rce-sans-pro/files/source-sans-variable-rest-normal.woff2

packages/font-source-sans-pro/files/static.css

packages/font-source-sans-pro/files/variable.css

packages/font-source-sans-pro/index.html

packages/font-source-sans-pro/poetry.lock