[Request] Improve support for custom number types

Nobody has claimed this yet.

In the builtin definitions, the `integer` type is defined as a child class of the `number` type:
```Lua
---@class integer : number
```
Despite that, it is still possible to write such code without any type-checking errors:
```Lua
---@param lives integer
local function printLives(lives)
print(lives)
end

---@type number
local lives = 9.0
printLives(lives)
```
I think this is fine, it makes sense.

However, this behaviour seems to be hardcoded in the extension.
Many of the functions from the API I use expect fixed-point values as parameters; such arguments are indicated by the `fixed` type, which is an alternate number type defined in much the same way as `integer`:
```Lua
---@class fixed : number
```
To represent literal fixed-point values, the API provides a unit contant which is used something like this:
```Lua
move(4*UNIT, 10*UNIT, UNIT*3/2) -- 40.0, 100.0, 1.5
```

Technically, `fixed` is just a regular number, however it is extremely helpful to the user to see this:
rather than this:

In older versions of the extension, this works perfectly fine; however, recent versions perform type-checking, which is a very useful feature but does not work nicely with the `fixed` type:
It looks like the extension considers `number * fixed` to be a `number`, which is a reasonable guess, but causes the error shown in the image above.

To solve this, I have thought of the following solutions:
1. Completely disabling type-checking. It works, obviously, but it would be preferable to not have to give up on such a useful feature.
2. Manually casting every literal value. This works too, but is extremely unpractical and results in bloating code.
3. Somehow make it so that `number * fixed` resolves to `fixed` instead of `number`. But that sounds like a very arbitrary solution to me.
4. Defining `fixed` as an alias: `---@alias fixed number`. It prevents the error from occuring, and the code is readable, but the completion/hover hints show "number" instead of "fixed", like in the second image.
5. Forking the extension to add a special case in the type checker.

There may also be other solutions I am not aware of, but my first impression is that solution 4 suits my usecase best, and would probably be perfect if there was some way to make an alias that behaves like in the first image (much like `typedef` in C, for instance).

Start with the builtin definitions for `integer` and `number`, then trace the type checker behavior for arithmetic involving custom number types such as `fixed`. Done means custom numeric types retain useful type-checking and completion or hover information without requiring casts, while preserving the existing `integer` behavior.

Written by the indexing model from the issue text.