Bug: Signed data values are handled incorrectly

[jeremy-code]

For SBYTE, SSHORT, SLONG, and SRATIONAL formats, .data returns a Uint8Array, which doesn't really make sense:

const signedArr1 = new Int8Array([-128, -1, 127])
new Uint8Array(signedArr1.buffer) // Uint8Array(3) [ 128, 255, 127 ]

Truthfully, using Uint8Array as the ONLY return type for .data probably shouldn't be done, and it probably should be done using the proper TypedArray for the different types, that being:

BYTE: Uint8Array
ASCII: Uint8Array(?) Or string
SHORT: Uint16Array
LONG: Uint32Array
RATIONAL: Uint32Array(?)
SBYTE: Int8Array
SSHORT: Int16Array
SLONG: Int32Array
SRATIONAL: Int32Array(?)
UNDEFINED: Uint8Array(?)

FLOAT and DOUBLE aren't handled in libexif, so those should be ignored.

[jeremy-code]

Fixed by 2d5ae99 with the addition of fromTypedArray and toTypedArray.

I declined to update .data directly because due to endianness/byte order, there is still some value in having things by Uint8Array and only using Int8Array for SBYTE doesn't seem very valuable. Since .fromTypedArray, .toTypedArray handles endianness based on the format automatically, this probably is a more useful implementation