Bump version to 0.15.0, update `CHANGELOG.md`

[github-actions[bot]]

Time Change: -638ms (3%)

Total Time: 18,230ms

Test name	Duration	Change
Serialization/Swift String to JavaScript	5,991ms	-326ms (5%)	✅

View Unchanged

| Test name | Duration | Change | | :--- | :---: | :---: | | Serialization/Write JavaScript number directly | 396ms | -18ms (4%) | | Serialization/Write JavaScript string directly | 451ms | -7ms (1%) | | Serialization/Swift Int to JavaScript | 5,941ms | -190ms (3%) | | Object heap/Increment and decrement RC | 5,452ms | -98ms (1%) |

[kateinoigakukun]

I think we should note that https://github.com/swiftwasm/JavaScriptKit/pull/187 introduced a new breaking change where Int64.jsValue and UInt64.jsValue are moved into JavaScriptBigIntSupport.

[j-f1]

(aside from @kateinoigakukun’s comment above)