make BcMath\Number::$value numeric-string

[schlndh]

Fixes https://phpstan.org/r/da494451-a17d-49f9-bcb7-5b2ce11219d2

It's covered by ReflectionProviderGoldenTest so I didn't add any other test:

1) PHPStan\Reflection\ReflectionProviderGoldenTest::test with data set "PROPERTY BcMath\Number::$value" ('PROPERTY BcMath\Number::$value', 'Readonly\nVisibility: public\...string')
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
 'Readonly
 Visibility: public
-Read type: string
-Write type: string'
+Read type: numeric-string
+Write type: numeric-string'

[ondrejmirtes]

Thank you!