Support Luau StrBuf type

[mxpv]

Relevant luau API:

// generic buffer manipulation

struct luaL_Strbuf
{
    char* p;   // current position in buffer
    char* end; // end of the current buffer
    lua_State* L;
    struct TString* storage;
    char buffer[LUA_BUFFERSIZE];
};
typedef struct luaL_Strbuf luaL_Strbuf;

// compatibility typedef: this type is called luaL_Buffer in Lua headers
// renamed to luaL_Strbuf to reduce confusion with internal VM buffer type
typedef struct luaL_Strbuf luaL_Buffer;

// when internal buffer storage is exhausted, a mutable string value 'storage' will be placed on the stack
// in general, functions expect the mutable string buffer to be placed on top of the stack (top-1)
// with the exception of luaL_addvalue that expects the value at the top and string buffer further away (top-2)

#define luaL_addchar(B, c) ((void)((B)->p < (B)->end || luaL_prepbuffsize(B, 1)), (*(B)->p++ = (char)(c)))
#define luaL_addstring(B, s) luaL_addlstring(B, s, strlen(s))

LUALIB_API void luaL_buffinit(lua_State* L, luaL_Strbuf* B);
LUALIB_API char* luaL_buffinitsize(lua_State* L, luaL_Strbuf* B, size_t size);
LUALIB_API char* luaL_prepbuffsize(luaL_Buffer* B, size_t size);
LUALIB_API void luaL_addlstring(luaL_Strbuf* B, const char* s, size_t l);
LUALIB_API void luaL_addvalue(luaL_Strbuf* B);
LUALIB_API void luaL_addvalueany(luaL_Strbuf* B, int idx);
LUALIB_API void luaL_pushresult(luaL_Strbuf* B);
LUALIB_API void luaL_pushresultsize(luaL_Strbuf* B, size_t size);

[mxpv]

d776b07