Accept f32/f64 as aliases for float32/float64.

In WebAssembly/component-model#277 there seems to be consensus emerging
to rename `float32`/`float64` to `f32`/`f64`. This PR just adds support
for parsing `f32`/`f64`, and changes nothing else for now, to start
preparing for this change.

crates/wit-parser/src/ast/lex.rs

@@ -285,8 +285,8 @@ impl<'a> Tokenizer<'a> {
                     "s16" => S16,
                     "s32" => S32,
                     "s64" => S64,
-                    "float32" => Float32,
-                    "float64" => Float64,
+                    "f32" | "float32" => Float32,
+                    "f64" | "float64" => Float64,
                     "char" => Char,
                     "resource" => Resource,
                     "own" => Own,