snippets: allow escaped shell vars like ${\1:?foo}

[AndrewDDavis]

The syntax for snippet placeholders is identical to some shell parameter expansions of the positional arguments in a script. If you want to put such shell expansions in a snippet, the snippet is mangled when it's inserted.

This PR fixes the problem by allowing shell snippets to have a syntax like ${\1:?message}, which doesn't match the pattern from snippet placeholders. After the code for identifying snippet placeholders is run, this call to gsub strips the backslash from the parameter expansion to produce the code that was actually intended.