capitalize() function

**Is your feature request related to a problem? Please describe.**
We are sometimes using capitalization for strings in bicep and I was always wondering, why there is no capitalize() function.
Currently (via bicep console):
> var test = 'test'
> var capitalized = '${toUpper(substring(test, 0, 1))}${substring(test, 1)}'
> capitalized
'Test'

**Describe the solution you'd like**
A dedicated capitalize() function

Start in the Bicep console by reproducing the current substring and toUpper workaround. Locate the existing string-function implementation and its tests, then add coverage for the requested capitalize() behavior and verify that the example produces 'Test'.

Written by the indexing model from the issue text.