Support using ---@class on rawset(_G, ...) to annotate the created global variable

[tomlau10]

closes #2862 (feature request), fixes #2863

---

This adds supports for using ---@class on rawset(_G, "key", {}) to specify the type of the created global variable:

---@class Test
rawset(_G, "Test", {})
print(Test) -- type: `Test`, instead of just `table`

Implementation details: #2862 (comment)
While implementing this, a bug related to rawset() was discovered and fixed as well: #2863 (comment)

[sumneko]

Thank you!