rustdoc-search: add search query syntax Fn(T) -> U

This is implemented, in addition to the ML-style one,
because Rust does it. If we don't, we'll never hear the end of it.

This commit also refactors some duplicate parts of the parser
into a dedicated function.

tests/rustdoc-js-std/parser-weird-queries.js

@@ -37,15 +37,6 @@ const PARSED = [
         userQuery: "a   b",
         error: null,
     },
-    {
-        query: 'a,b(c)',
-        elems: [],
-        foundElems: 0,
-        original: "a,b(c)",
-        returned: [],
-        userQuery: "a,b(c)",
-        error: "Expected `,`, `:` or `->`, found `(`",
-    },
     {
         query: 'aaa,a',
         elems: [