Symplify naming of the basic identifiers in the BNF grammar

[zafarkhaja]

According to the BNF grammar there are 3 basic types of identifiers: alphanumeric, numeric and "digital". The symbols are named as <alphanumeric identifier>, <numeric identifier> and digital id... no, just <digits>, respectively. So maybe, for the sake of brevity and consistency, we should rename the first two to just alphanumerics and numerics?

This is what it would look like as a result

...
<major> ::= <numerics>
<minor> ::= <numerics>
<patch> ::= <numerics>
...
<pre-release identifier> ::= <alphanumerics> | <numerics>
<build identifier> ::= <alphanumerics> | <digits>
...