Add digits argument to String::num_scientific and fix serializing

[aaronfranke]

Supersedes PR #86951, fixes #78204.

This PR adds a digits argument to String::num_scientific and various Variant methods, and uses this to serialize numbers with more precision.

We need to serialize with more precision to ensure that a serialized number can be deserialized into the same number. For example, for the number 123456789, the closest 32-bit float is 123456792. In master this is serialized as 1.23457e8, which becomes 123457000, over 200 off from the closest 32-bit float. With this PR, if a 32-bit float, it will be serialized with 9 digits as 123456792, which can be read back as exactly 123456792. 32-bit floats have 6 reliable digits, but 9 are needed to serialize to decimal in order to read back with full precision.

For an example with 64-bit floats, I have 1.234567898765432123456789e30 included in the test cases. The closest 64-bit float is 1.23456789876543218850569440461e30 (differs at the 8 which used to be a 2). This gets serialized as 1.2345678987654322e+30 which is deserialized to exactly 1.23456789876543218850569440461e30. 64-bit floats have 14 reliable digits, but 17 are needed to serialize to decimal in order to read back with full precision.

Note that this will result in more decimals being printed unnecessarily in some cases. For example, 3.4 becomes 3.4000001 as a 32-bit float and 3.3999999999999999 as a 64-bit float. A better but inefficient algorithm would be to keep increasing the digits until the value matches, but this would require constantly re-writing and re-parsing. I am not sure how this can be improved while still being efficient, but I am sure it's possible. If somebody wants to investigate how this can be improved before this PR is merged, that would be welcome.

Note that this does not affect the version bound to GDScript, which uses 6 digits just like it already does in master. I tried adding the argument to the bindings and it seems to break GDExtension compatibility.

Note that writing the unreliable digits should not be done when printing to the user, this is just meant for serializing.

Note that the docs have special code that uses 6 digits, since those are intended to be displayed to the user.

[dalexeev]

Note that get_construct_string() is also used in modules/gdscript/gdscript_editor.cpp.