Produce valid Cadence string literals when formatting strings

[turbolent]

Description

A Cadence developer might want to generate a valid Cadence string literal from a Go string.

Improve the string value formatting function format.String to do so.

Ref: https://axiomzen.slack.com/archives/CTGAW0TM2/p1624060674096500

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• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[turbolent]

I'm not 100% sure if we need this

[codecov-commenter]

Codecov Report

Merging #1023 (17114f6) into master (29f4ae1) will increase coverage by 0.01%.
The diff coverage is 89.28%.

@@            Coverage Diff             @@
##           master    #1023      +/-   ##
==========================================
+ Coverage   75.55%   75.56%   +0.01%     
==========================================
  Files         267      267              
  Lines       32968    32995      +27     
==========================================
+ Hits        24908    24932      +24     
- Misses       6937     6940       +3     
  Partials     1123     1123              

Flag	Coverage Δ
unittests	75.56% <89.28%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
runtime/format/string.go	89.28% <89.28%> (-10.72%)	⬇️

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