customcommands: align frontend length display with backend validation

[jo3-l]

The backend validation for custom command response length checks the number of runes (that is, Unicode code points) in the input:

yagpdb/customcommands/customcommands.go

Line 157 in b9dabf9

combinedSize += utf8.RuneCountInString(v)

However, the frontend character counter reads the length field of the response JavaScript String, which measures UTF-16 code units. The number of Unicode code points is not necessarily equal to the number of UTF-16 code units, occasionally producing a confusing discrepancy between frontend and backend. To demonstrate the issue, create a new custom command and run the following in console:

$('.cc-editor').val('😨'.repeat(5001)); // 😨 is encoded as 2 UTF-16 code units but is one code point
onCCChanged(); 

The character counter displays 10002/10000 (seemingly over the limit!) but this custom command can still be saved without error, since the backend validation only sees 5001 runes.

Fix this discrepancy by counting Unicode code points in the frontend as well.