Fix the parser to allow multiple strings one after the other

[GuptaManan100]

Description

This PR fixes the bug described in #15075.
According to the ANSI standard, strings that follow one after the other, should be concatenated together. We see the same behaviour in MySQL.
This PR replicates this behavior in Vitess as well. We now allow multiple strings one after the other, and like MySQL we concatenate them. A new precedence rule had to be introduced in the parser to remove the shift-reduce conflicts caused by the initial changes to the grammar.

Although we parse the queries now and our results are going to match MySQL, we are not making the effort to replicate the weird field name that MySQL has. i.e. -
MySQL output -

mysql [localhost:8032] {msandbox} ((none)) > select 'b' 'a' 'c';
+-----+
| b   |
+-----+
| bac |
+-----+
1 row in set (0.00 sec)

Vitess output -

mysql [localhost:8032] {vitess} ((none)) > select 'b' 'a' 'c';
+-----+
| bac |
+-----+
| bac |
+-----+
1 row in set (0.00 sec)

Related Issue(s)

• Fixes Bug Report: Vitess doesn't parse multiple strings #15075

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: 548 lines in your changes are missing coverage. Please review.

Comparison is base (eddb39e) 47.29% compared to head (8dd4678) 47.69%.
Report is 77 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/evalengine/cached_size.go	0.00%	41 Missing ⚠️
go/vt/vtgate/planbuilder/operators/delete.go	0.00%	32 Missing ⚠️
.../vtgate/planbuilder/operators/delete_with_input.go	0.00%	31 Missing ⚠️
.../vtgate/planbuilder/operators/horizon_expanding.go	0.00%	31 Missing ⚠️
go/vt/vtctl/workflow/traffic_switcher.go	0.00%	25 Missing ⚠️
go/vt/vtgate/engine/delete_with_input.go	53.70%	23 Missing and 2 partials ⚠️
...tgate/planbuilder/operators/aggregation_pushing.go	0.00%	25 Missing ⚠️
go/vt/mysqlctl/backupengine.go	0.00%	24 Missing ⚠️
go/vt/mysqlctl/builtinbackupengine.go	4.54%	21 Missing ⚠️
...vt/vtgate/planbuilder/operators/queryprojection.go	0.00%	21 Missing ⚠️
... and 55 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15076      +/-   ##
==========================================
+ Coverage   47.29%   47.69%   +0.39%     
==========================================
  Files        1137     1155      +18     
  Lines      238684   240151    +1467     
==========================================
+ Hits       112895   114542    +1647     
+ Misses     117168   117010     -158     
+ Partials     8621     8599      -22     

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[systay]

this is where I found the issue

[GuptaManan100]

❤️