jlexer: enforce RFC 8259 number grammar (no leading zeros)

[omkhar]

Problem

fetchNumber accepts number literals that violate the RFC 8259 number grammar — leading zeros ({"i":01} → 1, 007 → 7, 0123 → 123 as decimal, -007 → -7) and a trailing dot ({"f":1.} → 1). encoding/json rejects all of these ("invalid character … after …").

Same parser-differential / JSON-interoperability class as #375: a strict validator rejects the document as malformed while easyjson silently accepts it and assigns a non-obvious value.

Reproduce (current master): {"i":01} decodes with a nil error into I=1; encoding/json rejects it.

Fix

Enforce the RFC 8259 number grammar in fetchNumber: reject a leading zero in the integer part followed by another digit, and require at least one digit after . and after e/E.

• jlexer/lexer.go: +44/-3, contained to fetchNumber.
• go test ./... passes (jlexer, gen, parser, buffer, tests).

Same rollout question as the control-char change: this tightens default parsing. If you'd prefer it opt-in (cf. -disallow_duplicate_fields, #375; -disallow_unknown_fields), happy to rework. Flagging the trade-off rather than assuming.