Add toString method

[patinthehat]

This PR adds the toString() and __toString() methods.

[codecov]

Codecov Report

Merging #2 (dfac355) into main (1bcb09a) will increase coverage by 0.93%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main       #2      +/-   ##
============================================
+ Coverage     92.91%   93.83%   +0.93%     
- Complexity       60       63       +3     
============================================
  Files             4        4              
  Lines           155      162       +7     
============================================
+ Hits            144      152       +8     
+ Misses           11       10       -1     

Impacted Files	Coverage Δ
src/CodeSnippet.php	90.39% <100.00%> (+1.73%)	⬆️

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