Blender Git Loki

Geometry Nodes: String manipulation nodes

This patch adds four new nodes to a new "Text" category:
- String Length: Outputs length of a string
- String Substring: Outputs part of a string
- Value to String: Converts a value to a string
- String Join: Concatenates multiple strings with a delimiter

The initial use case of these nodes is the upcoming string to curve
node. However, they could also be used to calculate dynamic attribute
names, or with string attributes in the future.

Differential Revision: https://developer.blender.org/D12532