Include corner-shape in CSS syntax

[tylerjmorg]

Include the CSS corner-shape in syntax. Referenced in https://drafts.csswg.org/css-borders-4/#corner-shaping and shipping in Chrome 139 https://chromestatus.com/feature/5357329815699456

[RedCMD]

should move PR to upstream grammar repo
https://github.com/microsoft/vscode-css/blob/main/grammars/css.cson