Update the formatter to support tabs in addition to spaces

[adamdehaven]

In #9 we added the MDC formatter, but it requires the downstream Monaco Editor to utilize insertSpaces: true and detectIndentation: false.

We should update the formatter to include an additional FormatterOptions property of insertSpaces: boolean, and refactor the implementation to allow inserting space or tab character(s) \t for indentation based on the value of the insertSpaces option.

The result would then look like this when registering the document format providers:

// formatter implementation
export const formatter = (content: string, { 
  tabSize = 2, 
  isFormatOnType = false,
+ insertSpaces = true,
}: FormatterOptions): string => {
  // ... formatter code
})

// Register formatter
monaco.languages.registerDocumentFormattingEditProvider('mdc', {
  provideDocumentFormattingEdits: model => [{
    range: model.getFullModelRange(),
    text: mdcFormatter(model.getValue(), {
      tabSize: TAB_SIZE,
+     insertSpaces: false,
    }),
  }],
})

// Register format on type provider
monaco.languages.registerOnTypeFormattingEditProvider('mdc', {
  // Auto-format when the user types a newline character.
  autoFormatTriggerCharacters: ['\n'],
  provideOnTypeFormattingEdits: model => [{
    range: model.getFullModelRange(),
    // We pass `true` to `isFormatOnType` to indicate formatOnType is being called.
    text: mdcFormatter(model.getValue(), {
      tabSize: TAB_SIZE,
      isFormatOnType: true,
+     insertSpaces: false,
    }),
  }],
})