KCL: Customizable per-arg and per-fn snippet values (#7156)

Before, the LSP snippet for `startProfile` was 

```
startProfile(%, at = [3.14, 3.14])
```

Now it's 

```
startProfile(%, at = [0, 0])
```

This is configured by adding a `snippet_value=` field to the stdlib macro. For example:


```diff
#[stdlib {
    name = "startProfile",
    keywords = true,
    unlabeled_first = true,
    args = {
        sketch_surface = { docs = "What to start the profile on" },
-       at = { docs = "Where to start the profile. An absolute point." },
+       at = { docs = "Where to start the profile. An absolute point.", snippet_value = "[0, 0]" },        tag = { docs = "Tag this first starting point" },
    },
    tags = ["sketch"]
}]
```

## Work for follow-up PRs

 - Make this work for KCL functions defined in KCL, e.g. [`fn circle`](36c8ad439d/rust/kcl-lib/std/sketch.kcl (L31-L32)) -- something like `@(snippet_value = "[0, 0]")` perhaps
 - Go through the stdlib and change defaults where appropriate

rust/kcl-derive-docs/src/tests.rs

@@ -40,6 +40,9 @@ fn test_args_with_refs() {
     let (item, mut errors) = do_stdlib(
         quote! {
             name = "someFn",
+            args = {
+                data = { docs = "The data for this function"},
+            },
         },
         quote! {
             /// Docs
@@ -65,6 +68,9 @@ fn test_args_with_lifetime() {
     let (item, mut errors) = do_stdlib(
         quote! {
             name = "someFn",
+            args = {
+                data = { docs = "Arg for the function" },
+            }
         },
         quote! {
             /// Docs
@@ -117,7 +123,8 @@ fn test_stdlib_line_to() {
         quote! {
             name = "lineTo",
             args = {
-                sketch = { docs = "the sketch you're adding the line to" }
+                data = { docs = "the sketch you're adding the line to" },
+                sketch = { docs = "the sketch you're adding the line to" },
             }
         },
         quote! {