feat(eslint-plugin-template): [i18n] add allowMarkupInContent option

[m-akinc]

Fixes #930

Added allowMarkupInContent option for i18n rule. Option defaults to true to maintain backwards compatibility. Setting to false causes rule violation whenever an element with the i18n attribute has HTML markup in its content.

Note that due to the way the AST parser handles ICUs (i.e. it escapes any markup in cases so that they are strings), we cannot report violations for markup inside the ICU cases.

[m-akinc]

@JamesHenry There haven't been any comments or review since I put this up a couple weeks ago. Just wanted to ping someone to make sure it isn't overlooked.

[JamesHenry]

Thanks @m-akinc!

I would prefer it if no existing tests were modified by this PR to prove that this change is backwards compatible. Please can you make the necessary updates?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.43%. Comparing base (977f0b9) to head (7db729b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1795      +/-   ##
==========================================
- Coverage   90.47%   90.43%   -0.04%     
==========================================
  Files         178      178              
  Lines        3361     3368       +7     
  Branches      540      542       +2     
==========================================
+ Hits         3041     3046       +5     
- Misses        185      186       +1     
- Partials      135      136       +1     

Flag	Coverage Δ
unittest	90.43% <100.00%> (-0.04%)	⬇️

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Files	Coverage Δ
packages/eslint-plugin-template/src/rules/i18n.ts	91.66% <100.00%> (-1.20%)	⬇️
...s/eslint-plugin-template/tests/rules/i18n/cases.ts	100.00% <100.00%> (ø)

[JamesHenry]

Thank you @m-akinc!