docs: fix BNF grammar to distinguish prerelease from build identifiers (#846)

## Summary

The BNF grammar in `README.md` and `range.bnf` incorrectly defined `pre`
and `build` as identical rules (`parts`), but they have different
requirements per the [SemVer 2.0.0
specification](https://semver.org/#spec-item-9):

- **Prerelease identifiers**: Numeric parts must not have leading zeros
- **Build metadata**: Allows any alphanumeric string including leading
zeros

### Current behavior (correctly implemented, but BNF was wrong)

```js
semver.valid('1.2.3-00')  // null (invalid - prerelease numeric with leading zero)
semver.valid('1.2.3+00')  // '1.2.3' (valid - build metadata allows any alphanumeric)
semver.valid('1.2.3-0')   // '1.2.3-0' (valid - single zero is fine)
```

### Changes

Updates the BNF grammar to distinguish between:
- `prepart` - uses `nr` (no leading zeros) or `alphanumid` (alphanumeric
identifier)
- `buildid` - any alphanumeric string including leading zeros

Also adds an explanatory note after the BNF block to clarify the
distinction.

Fixes #665

## Test plan

- [x] Verified the implementation behavior matches the updated BNF
- [x] Confirmed alignment with [SemVer 2.0.0 spec item
9](https://semver.org/#spec-item-9)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: abhu85

README.md

@@ -399,12 +399,19 @@ nr         ::= '0' | ['1'-'9'] ( ['0'-'9'] ) *
 tilde      ::= '~' partial
 caret      ::= '^' partial
 qualifier  ::= ( '-' pre )? ( '+' build )?
-pre        ::= parts
-build      ::= parts
-parts      ::= part ( '.' part ) *
-part       ::= nr | [-0-9A-Za-z]+
+pre        ::= prepart ( '.' prepart ) *
+prepart    ::= nr | alphanumid
+build      ::= buildid ( '.' buildid ) *
+alphanumid ::= ( ['0'-'9'] ) * [-A-Za-z] [-0-9A-Za-z] *
+buildid    ::= [-0-9A-Za-z]+
 ```
 
+Note: Prerelease identifiers (`pre`) use `nr` for numeric parts, which
+disallows leading zeros (e.g., `1.2.3-00` is invalid). Build metadata
+identifiers (`build`) allow any alphanumeric string including leading
+zeros (e.g., `1.2.3+00` is valid). This matches the
+[SemVer 2.0.0 specification](https://semver.org/#spec-item-9).
+
 ## Functions
 
 All methods and classes take a final `options` object argument.  All

range.bnf

@@ -10,7 +10,8 @@ nr         ::= '0' | [1-9] ( [0-9] ) *
 tilde      ::= '~' partial
 caret      ::= '^' partial
 qualifier  ::= ( '-' pre )? ( '+' build )?
-pre        ::= parts
-build      ::= parts
-parts      ::= part ( '.' part ) *
-part       ::= nr | [-0-9A-Za-z]+
+pre        ::= prepart ( '.' prepart ) *
+prepart    ::= nr | alphanumid
+build      ::= buildid ( '.' buildid ) *
+alphanumid ::= ( [0-9] ) * [A-Za-z-] [-0-9A-Za-z] *
+buildid    ::= [-0-9A-Za-z]+