[firebase-analytics] Allow property names to contain numbers

[frantic]

This should bring the check more inline with the comment in Analytics.ts

Why

Fixes #8390 and adds the property name to the error message for easier debugging.

How

Fixed the regex.

Test Plan

'test'.match(/^[A-Za-z][[A-Za-z_\d]*$/);    // OK
'_test'.match(/^[A-Za-z][[A-Za-z_\d]*$/);   // Fail
'1_test'.match(/^[A-Za-z][[A-Za-z_\d]*$/);  // Fail
'a1_test'.match(/^[A-Za-z][[A-Za-z_\d]*$/); // OK

[IjzerenHein]

Awesome, thanks for this contribution!

Can you add an entry to CHANGELOG.md in the package as well?

[IjzerenHein]

I think we can also get rid of this line now

Suggested change

userPropertyName[0] === '_' ||

[frantic]

#8516 is a more complete solution.