Add sync package

[komuw]

• This has a more intuitive Waitgroup.

import "github.com/komuw/ong/sync"

wg, ctx := sync.New(context.Background(), 2)
_ = ctx

err := wg.Go(
    func() error {
        return nil
    },
   )

_ = err

[codecov-commenter]

Codecov Report

Attention: 38 lines in your changes are missing coverage. Please review.

Comparison is base (cccd396) 73.77% compared to head (71a9e56) 73.70%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #387      +/-   ##
==========================================
- Coverage   73.77%   73.70%   -0.08%     
==========================================
  Files          48       49       +1     
  Lines        5651     5753     +102     
==========================================
+ Hits         4169     4240      +71     
- Misses       1194     1220      +26     
- Partials      288      293       +5     

Files	Coverage Δ
middleware/client_ip.go	100.00% <100.00%> (ø)
middleware/middleware.go	100.00% <100.00%> (ø)
middleware/session.go	65.00% <100.00%> (+0.59%)	⬆️
sess/sess.go	78.87% <100.00%> (+0.61%)	⬆️
internal/clientip/client_ip.go	72.48% <0.00%> (-0.64%)	⬇️
config/config.go	64.74% <68.42%> (+0.16%)	⬆️
middleware/cors.go	92.30% <40.00%> (-2.70%)	⬇️
sync/sync.go	88.40% <88.40%> (ø)
cookie/cookie.go	73.77% <60.97%> (-3.95%)	⬇️

... and 1 file with indirect coverage changes

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