Replace fse with AzExtFsUtils where possible

[nturinski]

I couldn't get rid of fse entirely unfortunately...

• There's a createWriteStream command in requestUtils that there's no equivalent in workspace.fs
• The gulpfile doesn't actually have access to the utils package because the gulpfile doesn't run with the extension as far as I know. Whenever one is resolved, we should be able to move fse-extra to a devDependency or replace with fs calls
• A couple of calls to copy had to have the "overwrite" property added to them. It was causing issues in the tests when the backup templates were being updated.

[bwateratmsft]

I couldn't get rid of fse entirely unfortunately...

• There's a createWriteStream command in requestUtils that there's no equivalent in workspace.fs

Can't we make our own leveraging fs.writeFile along with an in-memory stream buffer?

[nturinski]

I couldn't get rid of fse entirely unfortunately...

• There's a createWriteStream command in requestUtils that there's no equivalent in workspace.fs

Can't we make our own leveraging fs.writeFile along with an in-memory stream buffer?

Yeah, I think we can but I wanted to do that separately just since it's a bit more of a complex change than just changing out some fs calls with VS Code's implementationm