mach: Configure uv using pyproject.toml (#39365)

We have been consolidating all of our Python configuration in
`pyproject.toml`, so we can move our one `uv` specific setting there as
well.

Testing: There is no easy way to write an automated test for this but I
confirmed it work by running `uv run --show-settings`.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

pyproject.toml

@@ -67,3 +67,6 @@ project-excludes = [
     "python/mach/**/*.py",
     "python/servo/mutation/**/*.py",
 ]
+
+[tool.uv]
+native-tls = true

uv.toml

@@ -1 +0,0 @@
-native-tls = true