Show definition information when hovering variable

Currently, when hovering (textDocument/hover) a variable, only the type is shown. I think it could be very useful if this would also show where the variable came from and if it is mut. For example:

fn foo(mut a: u32) {
    let b = 2i32;
    a;
    b;
}

Hovering a currently shows:

`u32`

but could show

`mut u32`
-------------
function parameter: `fn foo(mut a: u32)`

and for b:

`i32`
--------------
variable: `let b = 2i32;`

This is inspired by clangd, which does this:

void foo(unsigned int a) {
  int b = 2;
  a;
  b;
}

### param `a`

---
Type: `unsigned int`

---
// In foo
unsigned int a

### variable `b`

---
Type: `int`
Value = `2`

---
// In foo
int b = 2

<- it is even inferring the value, which is pretty cool

Start by tracing the textDocument/hover entry point and compare the current variable hover output with the examples in the issue. Determine how declaration information and mutability are represented, then verify that hovering variables shows their type, mutability, and declaration context without losing the existing type display.