Add bazel.completion.queryScope to scope completion target query

## Summary

Add a new `bazel.completion.queryScope` configuration option that allows users to limit the Bazel query used by the completion provider to specific package patterns, instead of always querying the entire workspace.

## Motivation

In large monorepos, the completion provider currently runs `kind('.* rule', ...)` which queries **all** targets in the workspace. This can be slow and return irrelevant results when a user only cares about a subset of packages.

## Proposed Change

Add a new setting `bazel.completion.queryScope` (type: `string[]`, default: `[]`) under the `bazel.completion` namespace.

- When **empty** (default), the existing behavior is preserved — the entire workspace is queried (`kind('.* rule', ...)`)
- When **non-empty**, the query is scoped to the specified patterns via union, e.g. `["//src/...", "//lib/..."]` → `kind('.* rule', //src/... + //lib/...)`

### Example configuration

```json
"bazel.completion.queryScope": ["//src/...", "//lib/..."]
```

## Difference from `bazel.commandLine.queryExpression`

| Setting | Type | Controls |
|---|---|---|
| `bazel.commandLine.queryExpression` | `string` | Workspace tree view & quick picker |
| `bazel.completion.queryScope` | `string[]` | BUILD file target auto-completion |

These two settings are independent and control different features.

Locate the completion provider and configuration definitions, starting with the existing `kind('.* rule', ...)` query and the related `bazel.commandLine.queryExpression` setting. Done means the new array setting defaults to empty, preserves the workspace-wide query when empty, and scopes completion queries to the configured package patterns when non-empty.

Written by the indexing model from the issue text.