Fix JSON5 number and escape grammar conformance

[gaoflow]

This tightens the number-literal and string-escape parsing in src/_decoder.pyx to match the JSON5 grammar. The parser currently both accepts inputs the spec forbids and rejects inputs it requires; the defects all cluster in the number scanner and the escape handler, so this fixes them as one class. The expected results below are from the json5 reference implementation (npm 2.2.3) and agree with stdlib json and dpranke's json5 where those apply.

Accepts inputs that should be rejected

input	current	reference
1_000, 1__0, 1_, 0x1_0, 1_e3	parsed, _ silently stripped	SyntaxError — JSON5 has no numeric separators
.e5, .e+5	0.0	SyntaxError — a leading . needs a fractional digit
5.8., .5.	5.8 / 0.5	SyntaxError — at most one ., never after the exponent
++7, +-.4	7 / -0.4	SyntaxError — a sign is only valid after e/E
0e, 0e+	0.0	SyntaxError — the exponent needs a digit
"\09", "\07", "\00"	"\x009", "\x007", …	SyntaxError — \0 before a digit is a legacy octal escape

Rejects inputs that should be accepted

input	current	reference
1e999, -1e999, 1e400	Json5EOF	Infinity, -Infinity (ECMAScript ToNumber)
0.e1	Json5EOF	0.0

"\0", "\0a", 1e-999 (→ 0.0), 1e308, .5, 1.e5, 0xFF, 01/00/007 (→ rejected) and every other currently-correct case are unchanged; the official json5-tests and JSONTestSuite suites stay green.

Fix

_decode_number_any now tracks the grammar explicitly — at most one ., at most one exponent, a sign only right after e/E, and a leading . must be followed by a digit — instead of silently dropping stray _/./sign characters. The leading-zero path (0.xxx / 0exxx) reuses the same scanner. _decode_double keeps fast_double_parser as the fast path and falls back to float() only for a grammatically valid token it rejects, which is exactly overflow (→ ±Infinity) and 0.e1. The \0 escape peeks the next code unit and rejects a following decimal digit.

Tests

scripts/run-conformance-test.py is a value-level table covering the whole class, added to CI and make test.

Build (Cython): git submodule update --init --recursive then pip install -e ..