Add less linter via npx

[xf-]

Why?
Add a linter to ensure consistent/unified code style.

Why this less liniter?
Other linter had nicer output, but was slower and no clear name of broken rule.

Config
Different config formats supported:
.lesshintrc (no highlight by default - json)
.lesshintrc.json (json with highlight - easy to find - currently used)
.lesshintrc.y?ml
package.json

Rules
Some rules from default stylish ruleset are disabled because of heavy usage. Maybe some can be removed, but changes some styles.

https://github.com/lesshint/lesshint/blob/master/lib/linters/README.md

{
  "idSelector": false, // is used
  "importPath": false, // less path _ import stuff  
  "importantRule": false, // often used and with heavy deep nesting hard to resolve
  "maxCharPerLine": false, // language & font stuff
  "propertyOrdering": false, // lot of work and not that important
  "qualifyingElement": false, 
  "spaceAroundComma": false, // looks broken and complains about correct syntax
  "stringQuotes": "double", // Singlequote are the default option
  "universalSelector": false, // Do not use * usage
  "zeroUnit": "no_unit" 
}

Exit codes for drone
https://github.com/lesshint/lesshint#exit-status-codes

[codecov-io]

Codecov Report

Merging #6936 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #6936      +/-   ##
=========================================
+ Coverage    41.4%   41.4%   +<.01%     
=========================================
  Files         441     441              
  Lines       59736   59736              
=========================================
+ Hits        24731   24732       +1     
+ Misses      31767   31765       -2     
- Partials     3238    3239       +1

Impacted Files	Coverage Δ
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️
modules/log/log.go	68.9% <0%> (-2.53%)	⬇️
routers/repo/view.go	43.03% <0%> (+1.01%)	⬆️
modules/avatar/avatar.go	100% <0%> (+18.75%)	⬆️

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[silverwind]

Good idea adding a linter. Generally, I came to prefer stylelint, maybe something we want to migrate to later.

[xf-]

@silverwind no problem with it. Should be easy to use a similar ruleset I tried a few linter for less (i normally work with yarn, sass and a gulp pipeline) and this one was the fastest. I searched based on links in lessc tool i found.

[silverwind]

FYI stylelint replacement up in #7305.