fix: backslash is not a valid quote escape

[olavloite]

The SimpleParser would accept backslash+quote as a valid escape mechanism
for a quote inside a quoted string. This is not correct for PostgreSQL,
and instead should be two equal quotes directly following each other.

[codecov]

Codecov Report

Merging #317 (f04538e) into postgresql-dialect (819a653) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@                   Coverage Diff                    @@
##             postgresql-dialect     #317      +/-   ##
========================================================
+ Coverage                 83.02%   83.04%   +0.02%     
- Complexity                 1450     1453       +3     
========================================================
  Files                       113      113              
  Lines                      5077     5085       +8     
  Branches                    653      657       +4     
========================================================
+ Hits                       4215     4223       +8     
  Misses                      650      650              
  Partials                    212      212              

Flag	Coverage Δ
all_tests	83.04% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
...oud/spanner/pgadapter/statements/SimpleParser.java	95.07% <100.00%> (+0.29%)	⬆️

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