feat(stdlib): add vim.base64 module (#25843)

Add base64 encode() and decode() functions to a vim.base64 module.

src/nvim/lua/base64.c

@@ -0,0 +1,65 @@
+#include <assert.h>
+#include <lauxlib.h>
+#include <lua.h>
+
+#include "nvim/base64.h"
+#include "nvim/lua/base64.h"
+#include "nvim/memory.h"
+
+static int nlua_base64_encode(lua_State *L)
+{
+  if (lua_gettop(L) < 1) {
+    return luaL_error(L, "Expected 1 argument");
+  }
+
+  if (lua_type(L, 1) != LUA_TSTRING) {
+    luaL_argerror(L, 1, "expected string");
+  }
+
+  size_t src_len = 0;
+  const char *src = lua_tolstring(L, 1, &src_len);
+
+  const char *ret = base64_encode(src, src_len);
+  assert(ret != NULL);
+  lua_pushstring(L, ret);
+  xfree((void *)ret);
+
+  return 1;
+}
+
+static int nlua_base64_decode(lua_State *L)
+{
+  if (lua_gettop(L) < 1) {
+    return luaL_error(L, "Expected 1 argument");
+  }
+
+  if (lua_type(L, 1) != LUA_TSTRING) {
+    luaL_argerror(L, 1, "expected string");
+  }
+
+  size_t src_len = 0;
+  const char *src = lua_tolstring(L, 1, &src_len);
+
+  const char *ret = base64_decode(src, src_len);
+  if (ret == NULL) {
+    return luaL_error(L, "Invalid input");
+  }
+
+  lua_pushstring(L, ret);
+  xfree((void *)ret);
+
+  return 1;
+}
+
+static const luaL_Reg base64_functions[] = {
+  { "encode", nlua_base64_encode },
+  { "decode", nlua_base64_decode },
+  { NULL, NULL },
+};
+
+int luaopen_base64(lua_State *L)
+{
+  lua_newtable(L);
+  luaL_register(L, NULL, base64_functions);
+  return 1;
+}