fix: Unescape JSON5 identifier values

[nzakas]

This pull request includes enhancements to the parse function in js/src/parse.js and corresponding tests in js/tests/parse.test.js. The main changes involve adding a new function to handle Unicode escape sequences in JSON5 identifiers and updating the parse function to use this new normalization.

Enhancements to parse function:

• js/src/parse.js: Added UNICODE_SEQUENCE regex and normalizeIdentifier function to convert Unicode escape sequences into their corresponding characters.
• js/src/parse.js: Updated the parse function to use normalizeIdentifier for processing identifiers.

Corresponding tests:

• js/tests/parse.test.js: Added a test case to ensure unquoted property names with Unicode escape sequences are correctly unescaped.

fixes #164