…#2290)

The following code now generates a Type Error:

```luau
type T = {
    read x: number
}

local foo: T = { x = 5 }
foo.x += 5
```

The flag `LuauLValueCompoundAssignmentVisitLhs` has been added, as well
as 1 test.

---------

Co-authored-by: ariel <aweiss@hey.com>

# Analysis

* Fix #1986
* Fix #1890
* Minor bugfixes and improvements

# Compiler

* Do not constant-fold strings that are longer than 4096 characters.
This helps to avoid pathalogical misoptimizations that could result in
the compiling bytecode growing very large.
* fix constant placement into the CHECK_BUFFER_LEN 'double source'
argument

---------

Co-authored-by: Vyacheslav Egorov <vegorov@roblox.com>
Co-authored-by: Ariel Weiss <arielweiss@roblox.com>