[vscode] Quick action Add custom word to .harper-dictionary.txt

I suggest adding a quick action to the vscode plugin so that it adds a new word directly to the .harper-dictionary.txt file.

Like cspell

Start in the VS Code plugin entry point and trace how quick actions and .harper-dictionary.txt are handled. Compare the requested flow with cspell’s “Add custom word” action, then verify that the action adds the selected word directly to the dictionary file.

Written by the indexing model from the issue text.