Substitute variables in extraFunction

[colinking]

The extraFunction option is the only option that accepts a path, but does not support variable substitution. This PR updates it to match configFile.

For context, we want to use the following VSCode config which requires extraFunction to be templatable.

"pgFormatter.configFile": "${workspaceFolder}/db/pg_format/pg_format.conf",
"pgFormatter.extraFunction": "${workspaceFolder}/db/pg_format/extra_functions.txt",

We already specify extra-function in our pg_format.conf, but that filepath is not resolved relative to the workspace root, so we need to override it with pgFormatter.extraFunction.

[bradymholt]

Looks good - thanks! I'll release this in just a few minutes.

[bradymholt]

https://github.com/bradymholt/vscode-pgFormatter/releases/tag/v1.24.0