Add default letter spacing for SvgTextTool

As per title,

my only concern is the range and step duplication here and https://invent.kde.org/graphics/krita/-/blob/fe20ff434cd7fded59c85b2d5a33eded3aca1fe9/plugins/tools/svgtexttool/SvgTextEditor.cpp#L1407-1408

should this be left as is or done something about?

Test Plan

• Pick SVG Text Tool
• Set letter spacing in tool options other than 0.00
• Create a text object
• Have non-0.00 letter spacing applied

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.