add: support for jinja ext

[thearchitector]

Many editors and tools look for the .html.jinja extension in order to provide formatting, color highlighting, styling, intellisense, etc. To support those use cases, don't try to append the .html extension to templates that already specify the .jinja extension, and also do autoescaping for .jinja files.

I'm also open to making it ignore only .html.jinja instead of just .jinja, but that would force folks into using .jinja.html instead, which feels like an incorrect ext.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (011d1ac) 97.57% compared to head (0f4e301) 97.57%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #406   +/-   ##
=======================================
  Coverage   97.57%   97.57%           
=======================================
  Files          66       66           
  Lines        6256     6256           
=======================================
  Hits         6104     6104           
  Misses        152      152           

Files Changed	Coverage Δ
blacksheep/server/rendering/jinja2.py	98.57% <100.00%> (ø)

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[RobertoPrevato]

Hi @thearchitector
I made .jinja the default extension for templates - I just released 2.0a10 which includes your contribution, and updated the MVC project template accordingly.