vscode: add a dictionary for cSpell

vscode: add a dictionary for cSpell

The quite useful cSpell extension allows VS Code to have "squiggly"
lines under spelling mistakes. By default, this would add too much
clutter, though, because so much of Git's source code uses words that
would trigger cSpell.

Let's add a few words to make the spell checking more useful by reducing
the number of false positives.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>