feat: add showPreciseDate setting

[tjkandala]

When set git.blame.showPreciseDate is true in user settings, all dates displayed by this extension will be absolute (as opposed to relative by default).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.03%. Comparing base (3021c9a) to head (66656fc).
⚠️ Report is 162 commits behind head on master.

Files with missing lines	Patch %	Lines
src/blame.ts	92.85%	1 Missing ⚠️
src/extension.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   47.58%   48.03%   +0.45%     
==========================================
  Files           4        4              
  Lines         124      127       +3     
  Branches       25       27       +2     
==========================================
+ Hits           59       61       +2     
- Misses         65       66       +1     

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[vovakulikov]

Nice work, LGTM. I left one minor comment, feel free to ignore it.