Add script to generate function map for static analysis tools

[alcaeus]

The newly added script generates function maps in the format that PHPStan and Psalm use. The function map is stored in scripts/functionmap.php. The script is invoked via a make target, ensuring that the extension is built to generate the correct map:

$ make generate-function-map

The script generates maps for all functions, classes, and interfaces in the MongoDB\BSON and MongoDB\Driver namespaces.

[alcaeus]

I made some more changes. Previously, the script always generated the function map for the installed extension version. There is now a make target called generate-function-map, which invokes the script the same way run-tests.php is invoked. This ensures that the extension is built before invoking the script with the latest extension version loaded. Additionally, the script now places a functionmap.php file in the scripts directory. This file is not committed to git, but it allows reformatting the code a little easier before copying it over to the files for PHPStan and Psalm.

[jmikola]

Some small suggestions but LGTM with some contributing docs added.