Make STTextView first responder when opening an empty file (CodeEdit C…

…odeEditApp#696) (CodeEditApp#144)

# Description

<!--- REQUIRED: Describe what changed in detail -->

* In `STTextViewController.swift`, the `setCursorPosition` function was
changed so that, if the currently open file is empty, the cursor is
automatically set to the first position and active. This behavior is
similar to other text editors e.g. VSCode.

A related PR is opened in CodeEdit, which works in tandem with this one,
so that newly-created files automatically open in a new editor tab.

# Related Issue

<!--- REQUIRED: Tag all related issues (e.g. * CodeEditApp#23) -->
* [CodeEditApp#696](CodeEditApp#696) from
CodeEdit.
* PR [CodeEditApp#1057](CodeEditApp#1057) from
CodeEdit.

# Checklist

<!--- Add things that are not yet implemented above -->
- [x] I read and understood the [contributing
guide](https://github.com/CodeEditApp/CodeEdit/blob/main/CONTRIBUTING.md)
as well as the [code of
conduct](https://github.com/CodeEditApp/CodeEdit/blob/main/CODE_OF_CONDUCT.md)
- [x] My changes generate no new warnings
- [x] My code builds and runs on my machine
- [x] I documented my code
- [x] Review requested

# Screenshots

Sources/CodeEditTextView/STTextViewController.swift

@@ -321,6 +321,16 @@ public class STTextViewController: NSViewController, STTextViewDelegate, ThemeAt
         var (line, column) = position
         let string = textView.string
         if line > 0 {
+            if string.isEmpty {
+                // If the file is blank, automatically place the cursor in the first index.
+                let range = NSRange(string.startIndex..<string.endIndex, in: string)
+                if let newRange = NSTextRange(range, provider: provider) {
+                    _ = self.textView.becomeFirstResponder()
+                    self.textView.setSelectedRange(newRange)
+                    return
+                }
+            }
+
             string.enumerateSubstrings(in: string.startIndex..<string.endIndex) { _, lineRange, _, done in
                 line -= 1
                 if line < 1 {