Upgrade UXStrings to version 0.7.0.

Add UXStrings 4 implementation (Ada.Containers.Vectors).
Use scenario variables in project files for choosing between UXStrings implementation UXS_VARIANT=UXS1..4.
Add Keep_Empty_Parts default parameter to Split.

index/ux/uxstrings/uxstrings-0.7.0+alpha-20240406.toml

@@ -0,0 +1,22 @@
+name = "uxstrings"
+description = "Unicode Extended Strings utilities"
+version = "0.7.0+alpha-20240406"
+tags = ["unicode", "dynamic", "string"]
+
+authors = ["Pascal Pignard"]
+maintainers = ["Pascal Pignard <[email protected]>"]
+maintainers-logins = ["Blady-Com"]
+website = "https://github.com/Blady-Com/UXStrings"
+
+licenses = "CECILL-C"
+
+project-files = ["lib_uxstrings.gpr"]
+
+[[depends-on]]
+gnat = ">=2021 | (>=12 & <2000)"
+simple_components = "^4.62.0"
+
+[origin]
+commit = "45a39c126944f2939ed2bee5fdb40d79e18bed35"
+url = "git+https://github.com/Blady-Com/UXStrings.git"
+