fix: correctly skip nested block comments

[olavloite]

Block comments with nested block comments were not correctly skipped.
The end tag of a nested block comment would be considered the end of the
entire comment block, instead of only reducing the block comment level.

Fixes b/236784930

[codecov]

Codecov Report

Merging #219 (8bf821c) into postgresql-dialect (9ec8e67) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@                  Coverage Diff                  @@
##             postgresql-dialect     #219   +/-   ##
=====================================================
  Coverage                 79.73%   79.73%           
- Complexity                 1041     1044    +3     
=====================================================
  Files                        91       91           
  Lines                      3543     3548    +5     
  Branches                    423      425    +2     
=====================================================
+ Hits                       2825     2829    +4     
  Misses                      553      553           
- Partials                    165      166    +1     

Flag	Coverage Δ
all_tests	79.73% <83.33%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
...cloud/spanner/pgadapter/utils/StatementParser.java	87.09% <83.33%> (-0.41%)	⬇️

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