VS Code variable resolution in @source annotations/relativity of paths

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### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Linux

### What is the issue affecting?

Diagnostics/Syntax Checking

### Expected Behaviour

Intuitively, I expected:

1. `---@source` annotations in meta files with relative paths to be resolved relative to the workspace
2. [VS Code variables](https://code.visualstudio.com/docs/reference/variables-reference#_predefined-variables) in the `@source` path to be resolved when resolving a path

### Actual Behaviour

1. `@source` paths are resolved relative to the meta file itself
2. VS Code variables in the path are not resolved

### Reproduction steps

1. Create `.vscode/luals/library/foo.d.lua` with:
```lua
---@meta
--- @class foo
_G.foo = {}

---@source grault/foo.lua:1
function bar() end
```
1. Create `baz.lua` with:
```lua
require("foo")
local quux = foo.bar()
```
3. In extension settings, set:
```json
{
"Lua.workspace.userThirdParty": ["${fileWorkspaceFolder}/LuaLS"]
}
```
> [!NOTE]
> The variable ${fileWorkspaceFolder} resolves correctly and adds the meta file to the list of search paths

4. Open `baz.lua` in VS Code editor
6. Right click `bar`
7. Select `Go to Definition`
8. Observe:
VS Code opens a non-existent source file tab to `.vscode/luals/library/grault/foo.lua`

### Additional Notes

This could be two separate issues, but they're tightly related -- one as a Documentation Issue that the relativity of the resolution of the path should be added to the docs, and two as a Bug/Feature Request that VS Code variables be resolved in the `@source` paths as they are in the settings

Attempting to work around this using [VS Code variables](https://code.visualstudio.com/docs/reference/variables-reference#_predefined-variables) has the variable unresolved and taken literally. For example, setting the `@source` path to `/${fileWorkspaceFolder}/grault/foo.lua:1` causes VS Code to open a tab to that literal string instead of the expected `/home/me/my_project/grault/foo.lua:1`

### Log File

_No response_

Reproduce the example with .vscode/luals/library/foo.d.lua, baz.lua, and the workspace.userThirdParty setting. Start by tracing how @source paths are resolved and how VS Code variables are expanded, then verify relative-path behavior and variable substitution against the expected Go to Definition target. Document the chosen path relativity if it remains unchanged.

Written by the indexing model from the issue text.