JSON: small optimization on number regex

• Shared context (JsonValue) to speed up monoline file opening with Qt5 (json of 1.9M: 45s -> 24s) (more detail: https://bugs.kde.org/show_bug.cgi?id=460578)
• A number cannot start with more than one 0, but each 0 was considered as Style_Decimal. Therefore, the regex was rewritten to allow multiple 0.
• remove insensitive="false" (default behavior)

Behaviour change (on an invalid format):

Before:

[000.10]
 | |
 Style_Decimal
   |
   Style_Float

After:

[000.10]
 |
 Style_Float