Add concatAST utility function

[vhenzl]

Cherry-picked from #929.

[codecov]

Codecov Report

Merging #930 (3c5934f) into master (2abac8c) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #930      +/-   ##
============================================
+ Coverage     94.16%   94.23%   +0.07%     
  Complexity       50       50              
============================================
  Files           117      117              
  Lines          9685     9684       -1     
============================================
+ Hits           9120     9126       +6     
+ Misses          565      558       -7     

Impacted Files	Coverage Δ
src/Utils/AST.php	93.05% <100.00%> (+0.16%)	⬆️
src/Utils/SchemaPrinter.php	98.96% <0.00%> (-0.07%)	⬇️
src/Language/Printer.php	99.25% <0.00%> (-0.03%)	⬇️
src/Language/Lexer.php	97.75% <0.00%> (ø)
src/Type/Definition/UnionType.php	95.45% <0.00%> (ø)
src/Utils/ASTDefinitionBuilder.php	90.00% <0.00%> (ø)
src/Type/Definition/InterfaceType.php	97.82% <0.00%> (ø)
src/Utils/BlockString.php
src/Language/BlockString.php	100.00% <0.00%> (ø)
src/Error/Error.php	94.89% <0.00%> (+0.05%)	⬆️
... and 3 more

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[vhenzl]

This should be good to go now.