[css] support css-variable completion in calc

Issue Type: <b>Bug</b> The css variable doesn't auto complete in the calc function in the form of var(--css-variable), rather it just stay like this calc(--css-variable), but it is supposed to be like this calc(var(--css-variable)). Thanks! VS Code version: Code 1.39.2 (6ab598523be7a800d7f3eb4d92d7ab9a66069390, 2019-10-15T15:35:18.241Z) OS version: Windows_NT x64 10.0.18362 <details><summary>Extensions (19)</summary> Extension|Author (truncated)|Version ---|---|--- ng-template|Ang|0.802.3 EditorConfig|Edi|0.14.2 vscode-npm-script|eg2|0.3.9 prettier-vscode|esb|2.3.0 php-intellisense|fel|2.3.12 auto-rename-tag|for|0.1.1 angular-essentials|joh|0.6.3 Angular2|joh|8.1.1 vscode-peacock|joh|3.1.6 vscode-language-babel|mgm|0.0.25 vscode-typescript-tslint-plugin|ms-|1.2.2 debugger-for-chrome|msj|4.12.1 angular2-inline|nat|0.0.17 incrementor|nms|0.1.0 angular-console|nrw|8.1.2 material-icon-theme|PKi|3.9.1 live-sass|rit|3.0.0 LiveServer|rit|5.6.1 JavaScriptSnippets|xab|1.7.2 (2 theme extensions excluded) </details> <!-- generated by issue reporter -->