feat: add optional reserveScrollBarGap

[huksley]

improved #475 for more information see original PR
Optional reserveScrollBarGap={true}

[codecov]

Codecov Report

Merging #484 (ca6b224) into master (61c3493) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head ca6b224 differs from pull request most recent head a19e9d0. Consider uploading reports for the commit a19e9d0 to get more accurate results

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
+ Coverage   95.23%   95.26%   +0.02%     
==========================================
  Files           6        6              
  Lines         189      190       +1     
  Branches       70       69       -1     
==========================================
+ Hits          180      181       +1     
  Misses          9        9              

Impacted Files	Coverage Δ
react-responsive-modal/src/index.tsx	100.00% <100.00%> (ø)
react-responsive-modal/src/useScrollLock.ts	100.00% <100.00%> (ø)

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[pradel]

@huksley thanks for the pr! Could you change the size so the CI pass please?

[huksley]

@pradel Done