chore: root repo vscode settings for rust-analyzer (#4556)

### Description

`rust-analyzer` only detects workspaces that are 1 directory deep in the
source tree, otherwise it has to be configured with the path to
workspaces. This PR checks in `.vscode` settings with paths to the rust
workspaces.

This also fixes the issue where someone checks out the monorepo to make
TS changes but can't commit because our husky precommit setups assumes
that rust workspaces have already been built (which `rust-analyzer` does
implicitly).

Noting that the problem is only there if the repo isn't opened using the
`mono.code-workspace` setting, which people I talked to don't use (other
than @paulbalaji)

.vscode/settings.json

@@ -0,0 +1,6 @@
+{
+    "rust-analyzer.linkedProjects": [
+        "./rust/main/Cargo.toml",
+        "./rust/sealevel/Cargo.toml",
+    ],
+}