feat(theme): support custom process of _date pipe

[cipchk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ng-alain/delon/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[cipchk]

Preview is ready!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.89%. Comparing base (6389ec1) to head (a08936d).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1822    +/-   ##
========================================
  Coverage   94.89%   94.89%            
========================================
  Files         315      315            
  Lines        8906     8908     +2     
  Branches     1789     1688   -101     
========================================
+ Hits         8451     8453     +2     
  Misses        381      381            
  Partials       74       74            

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