allow modify poll_delay_ms argument using an environment variable

[joelpelaez]

related #302

Add a new environment variable named WATCHFILES_POLL_DELAY_MS can allow change poll_delay_ms argument value from functions watch and awatch.

It helps when other libraries uses watchfiles and poll delay value can't be changed directly using thrid party library interfaces.

[samuelcolvin]

otherwise LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.06%. Comparing base (a81a7c6) to head (67fe6ed).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #303      +/-   ##
==========================================
- Coverage   99.51%   99.06%   -0.45%     
==========================================
  Files           6        6              
  Lines         416      430      +14     
  Branches       77       79       +2     
==========================================
+ Hits          414      426      +12     
- Misses          1        2       +1     
- Partials        1        2       +1     

Files with missing lines	Coverage Δ
watchfiles/main.py	96.80% <100.00%> (-1.40%)	⬇️

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[samuelcolvin]

thanks so much.