Casing transforms in snippets only support Latin characters

[dmitrivMS]

Testing #233798

Use the following snippet:

"testSnippet": {
  "scope": "javascript,typescript",
  "prefix": "testSnippet",
  "body": "${1} -> ${1/(.*)/${1:/upcase} ${1:/downcase} ${1:/camelcase} ${1:/pascalcase} ${1:/kebabcase} ${1:/snakecase}/}"
}

Give it input oneTwo you will get oneTwo -> ONETWO onetwo oneTwo OneTwo one-two one_two which is correct.
Give it input одинДва you will get одинДва -> ОДИНДВА одиндва одинДва одинДва одинДва одиндва which is incorrect as non-Latin letters are not handled correctly (in this case Russian alphabet, but code only cares about a-z).

We should probably use Unicode definition of words if possible to perform the case transforms.