vscode.TextDocument.getWordRangeAtPosition
https://code.visualstudio.com/api/references/vscode-api#TextDocument
Get a word-range at the given position.
vscode.Rangeを返す
A range spanning a word, or undefined.
It is also possible to provide a custom regular expression.
Note 1: A custom regular expression must not match the empty string and if it does, it will be ignored.
正規表現で表したwordに該当するrangeを取得できる（ないこともある）