Use `jsonc` syntax highlighting for github

[S.Y. Lee]

Technically, I figured out the reason why we are getting 'red' for comments,

It is because the grammar of json https://www.json.org/json-en.html does not allow any comments,
however, this is not an issue with most of the JSON parsers out there.

sympy_benchmarks/asv.conf.json

Lines 16 to 32 in eec7104

 // List of branches to benchmark. If not provided, defaults to "master" 
 // (for git) or "tip" (for mercurial). 
 // "branches": ["master"], // for git 
 // "branches": ["tip"],    // for mercurial 
  
 // The DVCS being used.  If not set, it will be automatically 
 // determined from "repo" by looking at the protocol in the URL 
 // (if remote), or by looking for special directories, such as 
 // ".git" (if local). 
 // "dvcs": "git", 
  
 // The tool to use to create environments.  May be "conda", 
 // "virtualenv" or other value depending on the plugins in use. 
 // If missing or the empty string, the tool will be automatically 
 // determined by looking for tools on the PATH environment 
 // variable. 
 "environment_type": "virtualenv", 

It is possible to fix this in .gitattributes

*.json linguist-language=jsonc