Bump version to 0.15.0, update CHANGELOG.md

[MaxDesiatov]

No description provided.

[github-actions]

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 #187 introduced a new breaking change where Int64.jsValue and UInt64.jsValue are moved into JavaScriptBigIntSupport.

[j-f1]

(aside from @kateinoigakukun’s comment above)