fix (@jitsu/js): Preserve anonymous ID as a string and avoid parsing

[dmeremyanin]

This PR addresses an issue where the __anon_id cookie was being parsed, potentially altering its value. The __anon_id must always be returned as a string to preserve its exact value.

Issue Example

Before the fix, parsing the cookie value 1724633695283.638279 with JSON.parse() would convert it to a JS number, which results in precision loss. In this case, the number would become 1724633695283.6382 due to the way JavaScript represents floating-point numbers.

After the fix, the __anon_id value is returned as a string without any change, preserving the exact value.