Add a setting to choose the character used by toggleBold

[dorian-marchal]

For italic, we can choose the wrapper char with markdown.extension.italic.indicator setting:

https://github.com/neilsustc/vscode-markdown/blob/e0649e30108e43f0f391b261afa3df43669409e1/package.json#L229-L236

It could be nice to have the same thing for bold (e.g. markdown.extension.bold.indicator) so we can toggleItalic *like this* and toggleBold __like this__.

[yzhang-gh]

Good for PR