fix(color): off-by-one for `INDEXED_COLORS` access

[RoloEdits]

Current implimentation sets the value right from the color tags value, e.g <color indexed="2", which is already zero-indexed.

https://github.com/MathNya/umya-spreadsheet/blob/3c34cb8d09c242c214aa33640378fd096f641e6e/src/structs/color.rs#L212-L215

The - 1 would then access the previous indexes color if it was <color indexed="1..", or return None if it was <color indexed="0".

The flipside to this is change is that the implimentation for set_value no longer needs to add 1. The tests for this have been modified for this change.

I added a single test with red color "RED" in A1

which results in an styles.xml with:

<font>
    <sz val="11.000000" />
    <color indexed="2" />
    <name val="Calibri" />
    <scheme val="minor" />
</font>

[MathNya]

@RoloEdits Thank you very much. We will merge three PRs.