[gdb/contrib] Add more separators in spellcheck.sh

[gdb/contrib] Add more separators in spellcheck.sh

Add two more separators in spellcheck.sh: colon and comma.

Doing so triggers the "inbetween->between" rule, which gives an incorrect
result.  Override this with "inbetween->between, in between, in-between" [1],
in a new file gdb/contrib/common-misspellings.txt.

Fix the following common misspellings:
...
everytime -> every time
sucess -> success
thru -> through
transfered -> transferred
inbetween -> between, in between, in-between
...

Verified with spellcheck.sh.  Tested on x86_64-linux.

[1] https://www.grammarly.com/blog/commonly-confused-words/in-between-or-inbetween/