hclwrite: fix space being added between interpolated items

[acburdine]

closes #65

• add missing edge case for two interpolated items next to each other
• add tests for both quote and heredoc cases

[codecov-io]

Codecov Report

Merging #77 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   65.74%   65.75%   +<.01%     
==========================================
  Files          97       97              
  Lines       10241    10243       +2     
==========================================
+ Hits         6733     6735       +2     
  Misses       3179     3179              
  Partials      329      329

Impacted Files	Coverage Δ
hclwrite/format.go	93.56% <100%> (+0.05%)	:arrow_up:

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[apparentlymart]

Thanks so much for working on this, @acburdine! I was about to dive into this myself, so I was pleasantly surprised to find this PR here when I went to look at #65!

I had expected that we'd need to make an exception where we suspend processing of spaces between TokenOQuote and TokenCQuote, but in retrospect what you did here makes sense and is much more straightforward, since we know that the only other token types that can come before or after interpolation sequences are TokenOQuote, TokenCQuote, and TokenQuotedLiteral.

This will also allow us to close out hashicorp/terraform#19744 once we update the vendored version of HCL2 to include this change.