Add an option for filename relative to workspace directory

[xulongwu4]

Hi, first of all let me say that I like this plugin very much. It adds a lot of flexibility to coc.nvim.

While adding some linter and formatter myself, I found some of them can take the relative file path with respect to the workspace directory. Right now this language server supports "%text", "%filename", "%file", "%filepath". Is it support to add support for file's relative path when rootPattern is specified? Thanks.

[iamcco]

Not support yet.

[ngmy]

I think the relative path is very useful when running linter over Docker. I am looking forward to it.

[iamcco]

%relativepath is supported