Moved C# SDK's packages ignore from SDK's .gitignore to project root (#4211)

# Description of Changes

Moves the `root/sdks/csharp/packages/` directory from the
`root/sdks/csharp/.gitignore` to `root/.gitignore`.
This is to resolve Issue #4207 , which was introduced during the DLLs
generation rework.

`/sdks/csharp/packages/` should be ignored by git, to prevent locally
generated DLLs from accidentally getting added to the repo.

**Root cause:**
When Unity imports a package by URL, it looks at the `.gitignore` list
and filters by it, causing the entire `packages` directory to not be
imported. In earlier versions of Unity (like `2022.3` where this was
initially tested), this was not a problem because Unity would allow us
to do a `dotnet restore`. In modern version of Unity, this is no longer
allowed as all imported packages are considered immutable.

Because the `/sdks/csharp/` is cloned into
`com.clockworklabs.spacetimedbsdk`, if we move at what level the
`root/sdks/csharp/packages/` is ignored at, we can prevent the
accidental inclusion of the DLLs from developers in the SpacetimeDB
repo, while ensuring `com.clockworklabs.spacetimedbsdk` does not contain
a reference to ignore the `packages` directory.

# API and ABI breaking changes

No API or ABI changes

# Expected complexity level and risk

1

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning
ways. -->

# Testing

- [X] Tested removing the `packages` directory from the `.gitignore`
list on a custom branch of `com.clockworklabs.spacetimedbsdk` to observe
the behavior in both Unity `6000.3.2f1` and `2002.3.62f2`.

Author: rekhoff

.gitignore

@@ -245,3 +245,7 @@ nul
 
 [Nn][Uu][Gg][Ee][Tt].[Cc][Oo][Nn][Ff][Ii][Gg]
 [Nn][Uu][Gg][Ee][Tt].[Cc][Oo][Nn][Ff][Ii][Gg].meta
+ 
+# csharp SDK packages
+/sdks/csharp/packages/
+/sdks/csharp/packages.meta

sdks/csharp/.gitignore

@@ -75,9 +75,4 @@ obj~
 # This is used for local paths to SpacetimeDB packages.
 [Nn][Uu][Gg][Ee][Tt].config
 [Nn][Uu][Gg][Ee][Tt].config.meta
-.idea/
-
-# Hydrated SDK DLLs (produced by `cargo ci dlls`)
-/packages/
-/packages.meta
-!/packages/.gitignore
+.idea/