docs: fix BNF grammar to distinguish prerelease from build identifiers

[abhu85]

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:

• 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)

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

• Verified the implementation behavior matches the updated BNF
• Confirmed alignment with SemVer 2.0.0 spec item 9

[wraithgar]

Had to go back and double check a few things there, just to be sure..

> semver.parse('1.2.0--pre').prerelease
[ '-pre' ]
> semver.parse('1.2.0+-build').build
[ '-build' ]

Yep, leading - is ok on those.