Implement RStatementRangeProvider

[DavisVaughan]

Joint with posit-dev/ark#85
Part of #1213
Part of #394

This currently has the behavior where if you hit CMD+Enter within a function definition (like on line 1 + 2 in the video) then it will send the entire function to the console rather than the current line (similar behavior occurs within a function call). RStudio does the opposite, but I actually don't mind this new behavior all that much, and the implementation is very simple for now. You can always opt out of this by doing a manual selection of 1 + 2 and sending just that to the console

Screen.Recording.2023-09-05.at.1.53.44.PM.mov

After this PR (both of which I'll look at):

• Small bug related to Bug in StatementRangeProvider usage #1231
• Tweak provideStatementRange() to allow | undefined all the way through, rather than catching and erroring on it here

  positron/src/vs/workbench/api/common/extHostLanguageFeatures.ts

  Lines 1587 to 1593 in 07925f0

  	const providerRange = await this._provider.provideStatementRange(document, position, token);
  	if (!providerRange) {
  	this._logService.debug(`No statement range from provider ` +
  	`for position ${position} in ${resource}`);
  	throw new Error(`Invalid statement range returned from provider ` +
  	`for ${position} in ${resource}`);
  	}