feat: add MemoryFileSystem module

.changeset/puny-queens-spend.md

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+---
+"effect": minor
+---
+
+Add the `MemoryFileSystem` module with an isolated in-memory `FileSystem` layer.

packages/effect/src/MemoryFileSystem.ts

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+/**
+ * In-memory implementation of Effect's `FileSystem` service.
+ *
+ * This module provides an isolated virtual POSIX filesystem for tests and
+ * programs that need filesystem behavior without host filesystem I/O. It
+ * supports files, directories, links, descriptors, temporary resources, globbing,
+ * and file watching through the standard `FileSystem` service.
+ *
+ * @since 4.0.0
+ */
+import type * as Effect from "./Effect.ts"
+import type * as FileSystem from "./FileSystem.ts"
+import * as internal from "./internal/memoryFileSystem.ts"
+import type * as Layer from "./Layer.ts"
+
+/**
+ * Creates a `FileSystem` service backed by a fresh in-memory volume.
+ *
+ * **When to use**
+ *
+ * Use when you need to construct and provide an isolated filesystem explicitly.
+ *
+ * @see {@link layer} for providing the service as a Layer.
+ * @category constructors
+ * @since 4.0.0
+ */
+export const make: Effect.Effect<FileSystem.FileSystem> = internal.make
+
+/**
+ * Provides an isolated in-memory implementation of `FileSystem.FileSystem`.
+ *
+ * **When to use**
+ *
+ * Use when you need an in-memory filesystem as a dependency for a test or program.
+ *
+ * **Gotchas**
+ *
+ * Reusing this layer value shares one volume through layer memoization. Construct
+ * a new layer or make the layer fresh when each consumer needs an isolated volume.
+ *
+ * @see {@link make} for constructing the service directly.
+ * @category layers
+ * @since 4.0.0
+ */
+export const layer: Layer.Layer<FileSystem.FileSystem> = internal.layer