External tag

[Sprexatura]

Adopt external-tag to serialized union values

[kanghyojun]

As I mentioned earlier, there are some style issues.

src/Nirum/Targets/Python.hs:850:1: too long line (107 chars)
src/Nirum/Targets/Python.hs:850:108: trailing (1) white space
src/Nirum/Targets/Python.hs:875:1: too long line (92 chars)
src/Nirum/Targets/Python.hs:876:57: trailing (1) white space
src/Nirum/Targets/Python.hs:884:15: trailing (4) white space

[kanghyojun]

@dahlia @Kroisse Could you all review the idea of this PR regardless of the failing of CI? We decided to support external tag by adding @external-tag annotation on record type. And maybe in the next minor version updates, we could support external tag as default.

[codecov[bot]]

Codecov Report

Merging #316 into master will increase coverage by 0.04%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   76.22%   76.26%   +0.04%     
==========================================
  Files          34       34              
  Lines        2902     2903       +1     
  Branches      194      196       +2     
==========================================
+ Hits         2212     2214       +2     
+ Misses        496      493       -3     
- Partials      194      196       +2

Impacted Files	Coverage Δ
src/Nirum/Targets/Python.hs	88.2% <100%> (+0.03%)	:arrow_up:
src/Nirum/Constructs/Docs.hs	94.73% <0%> (ø)	:arrow_up:
src/Nirum/Constructs/Annotation/Internal.hs	77.27% <0%> (ø)	:arrow_up:
src/Nirum/Constructs/Identifier.hs	98.14% <0%> (+1.85%)	:arrow_up:

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