feat(lsp): inlay hints of variable types and command params (#14802)

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# Description
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This PR adds inlay hints of variable types and parameter names to
lsp-server
<img width="547" alt="image"
src="https://github.com/user-attachments/assets/07a0dd84-5ecc-47df-a8a7-732631715662"
/>

Some design choices I made:
* for composite types like `record<foo: <record ...>>`, only a short
name displayed. Full signature already available through `hover`
* only parameter names of user defined commands are returned, feels too
much distraction if enabled for all builtins
* some information are lost in flattened expressions, so I implemented
my AST traversing functions, which may seem unnecessary, but I can't
find alternatives from the existing code.
* another minor change: added a line separator to current hover markdown
message.

# User-Facing Changes
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Users who think this feature annoying now have to manually turn it off
(or config the lsp client capabilities).

# Tests + Formatting
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tests/fixtures/lsp/hints/param.nu

@@ -0,0 +1,13 @@
+def cmd [
+  a1
+  a2
+  --flag (-f)
+  a3? # arg3
+  a4?
+  ...arg_rest
+] { }
+
+ls | cmd 1 $nu -f (
+  cmd 1
+  2
+) ...[(cmd 1 2)]

tests/fixtures/lsp/hints/type.nu

@@ -0,0 +1,8 @@
+let int = 1
+let str = "hello"
+let bool = true
+let float = 1.0
+let list = [1 2 3]
+let record = {}
+let closure = {|x| $x }
+let ignored: int = 5