fix(edit): Force indexmap std support

`indexmap` 1.9.2 and before (the latest) auto-detect the use of std.  If
it fails, `IndexMap` does not default the third generic parameter,
causing builds to fail.  In `indexmap` master, this is fixed by
defaulting the `std` feature.

Fixes #524

crates/toml_edit/Cargo.toml

@@ -47,7 +47,7 @@ serde = ["dep:serde", "toml_datetime/serde", "dep:serde_spanned"]
 unbounded = []
 
 [dependencies]
-indexmap = "1.9.1"
+indexmap = { version = "1.9.1", features = ["std"] }
 winnow = "0.3.0"
 serde = { version = "1.0.145", optional = true }
 kstring = { version = "2.0.0", features = ["max_inline"], optional = true }