The BindingIdentifier of LexicalDeclaration should be allowed to start with Unicode escape

[JLHwung]

let \u0061;
let in\u0061;
let in𝐬𝐭𝐚𝐧𝐜𝐞𝐨𝐟;
let 𝐢𝐧;

Acorn 8.0.4 throws: https://astexplorer.net/#/gist/68b52ba3dff393e8709d4778dad5b868/449ca9bb2f2428319e0bfc547a80d1d9a514b289

Context: babel/babel#13323

[marijnh]

Good catch. Attached patch improves the code that recognizes contextual let keywords to take these into account.