Fix multiple cursor offset after inserting opening pair

[NiceSieve]

Code was editing the region before comparing it to the previously inserted region.
Also add doc to matching_pair_direction function.

This closes #1374

[NiceSieve]

Am writing a regression test right now, just want the CI checks in parallel :)

[codecov-commenter]

Codecov Report

Merging #1423 (c16cf5a) into master (3ce1306) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           master   #1423      +/-   ##
=========================================
+ Coverage    6.10%   6.16%   +0.06%     
=========================================
  Files         122     122              
  Lines       49317   49289      -28     
=========================================
+ Hits         3013    3041      +28     
+ Misses      46304   46248      -56     

Impacted Files	Coverage Δ
lapce-core/src/syntax/util.rs	20.28% <ø> (+2.89%)	⬆️
lapce-core/src/editor.rs	29.86% <100.00%> (+1.45%)	⬆️
lapce-data/src/proxy.rs	0.00% <0.00%> (ø)
lapce-data/src/update.rs	0.00% <0.00%> (ø)
lapce-proxy/src/plugin/lsp.rs	0.00% <0.00%> (ø)
lapce-core/src/cursor.rs	5.54% <0.00%> (+0.27%)	⬆️

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