Add buffer library

Author: Sainan

docs/Runtime Environment/Buffer.md

@@ -0,0 +1,38 @@
+The buffer class — available via `require` — is a fast intermediate storage for incrementally-constructed strings.
+
+---
+### `buffer.new`
+
+Creates a new buffer instance.
+
+---
+### `buffer.append`
+
+Appends a string to a buffer instance.
+
+#### Parameters
+
+1. The buffer instance.
+2. The string to append.
+
+---
+### `buffer.tostring`, `__tostring`
+
+Converts the buffer into a string.
+
+#### Parameters
+
+1. The buffer instance.
+
+#### Returns
+
+A string.
+
+```pluto
+local buffer = require "pluto:buffer"
+
+local buff = new buffer()
+buff:append("Hello,")
+buff:append(" world!")
+print(buff) --> Hello, world!
+```