terminal: Make CursorShape configurable (#18530)

This builds on top of @Yevgen's #15840 and combines it with the settings
names introduced in #17572.

Closes #4731.

Release Notes:

- Added a setting for the terminal's default cursor shape. The setting
is `{"terminal": {"cursor_shape": "block"}}``. Possible values: `block`,
`bar`, `hollow`, `underline`.

Demo:


https://github.com/user-attachments/assets/96ed28c2-c222-436b-80cb-7cd63eeb47dd

assets/settings/default.json

@@ -671,6 +671,18 @@
     //  3. Always blink the cursor, ignoring the terminal mode
     //         "blinking": "on",
     "blinking": "terminal_controlled",
+    // Default cursor shape for the terminal.
+    //  1. A block that surrounds the following character
+    //     "block"
+    //  2. A vertical bar
+    //     "bar"
+    //  3. An underline that runs along the following character
+    //     "underscore"
+    //  4. A box drawn around the following character
+    //     "hollow"
+    //
+    // Default: not set, defaults to "block"
+    "cursor_shape": null,
     // Set whether Alternate Scroll mode (code: ?1007) is active by default.
     // Alternate Scroll mode converts mouse scroll events into up / down key
     // presses when in the alternate screen (e.g. when running applications