Fix and restyle menu on code line (#15913)

* Fix and restyle menu on code line

* fix multiline and more tweaks

* move to separate files

* remove has-context-menu class

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/index.less

@@ -11,6 +11,7 @@
 @import "./features/projects.less";
 @import "./markup/content.less";
 @import "./markup/mermaid.less";
+@import "./code/linebutton.less";
 
 @import "./chroma/base.less";
 @import "./chroma/light.less";