refactor(types): Hide .types imports behind TypeGuard

[tony]

This avoids the need for typing-extensions as an official dependency and fixes #412.

IF this way doesn't work, merge @Jasha10's approach at #413

[codecov]

Codecov Report

Merging #414 (435a26c) into master (18a73e4) will decrease coverage by 0.46%.
The diff coverage is 100.00%.

❗ Current head 435a26c differs from pull request most recent head 325af1c. Consider uploading reports for the commit 325af1c to get more accurate results

@@            Coverage Diff             @@
##           master     #414      +/-   ##
==========================================
- Coverage   84.00%   83.55%   -0.46%     
==========================================
  Files          17       16       -1     
  Lines         938      912      -26     
  Branches      168      168              
==========================================
- Hits          788      762      -26     
  Misses        100      100              
  Partials       50       50              

Impacted Files	Coverage Δ
src/vcspull/validator.py	36.84% <ø> (-3.16%)	⬇️
tests/test_config.py	96.87% <ø> (-0.10%)	⬇️
src/vcspull/config.py	84.55% <100.00%> (-0.12%)	⬇️
tests/fixtures/example.py	100.00% <100.00%> (ø)
tests/test_sync.py	90.14% <100.00%> (-0.14%)	⬇️

... and 1 file with indirect coverage changes

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