Unify Printer and SchemaPrinter output for block strings

[vhenzl]

Adds StringBlock::print() method, an equivalent of printBlockString() function from graphql-js. It's used to print string blocks in Printer and SchemaPrinter.

Relevant changes in the reference implementation:

• Unify 'print' and 'printSchema' output for block strings  graphql/graphql-js#1751
  (commits graphql/graphql-js@ded3261 and graphql/graphql-js@961dab3)
• printSchema: don't break description lines graphql/graphql-js#2223

The behaviour matches graphql-js v15 (v16 removes the indentation inside of block strings: graphql/graphql-js@45e33ce)

[vhenzl]

Ah, I didn't realise symfony/polyfill-php80 is just a transitive dev dependency :/

[codecov]

Codecov Report

Merging #938 (1b2f90d) into master (88f9f49) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #938      +/-   ##
============================================
- Coverage     94.23%   94.23%   -0.01%     
  Complexity       50       50              
============================================
  Files           117      117              
  Lines          9679     9674       -5     
============================================
- Hits           9121     9116       -5     
  Misses          558      558              

Impacted Files	Coverage Δ
src/Language/BlockString.php	100.00% <100.00%> (ø)
src/Language/Printer.php	99.25% <100.00%> (-0.03%)	⬇️
src/Utils/SchemaPrinter.php	98.94% <100.00%> (-0.09%)	⬇️

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