Add configurable cursor shape via settings.cursor_shape

[xsa-dev]

Summary

Add a new settings.cursor_shape option to configure the terminal cursor shape used by iamb.

Supported values:

• default
• block
• line
• underline

Changes

• add CursorShape config enum with serde parsing
• add cursor_shape to Tunables / TunableValues (including merge + default handling)
• apply selected shape in TTY setup using crossterm::cursor::SetCursorStyle
• restore terminal cursor style to default on exit
• document the new setting in docs/iamb.5
• add cursor_shape = \"default\" to config.example.toml
• add unit test coverage for parsing (test_parse_cursor_shape)

Motivation

Closes a long-standing request to support non-block cursors and allows users to match their terminal/editor preferences.

Closes #197

[VAWVAW]

This works as described but I'm not sure that this is the best way to implement it. I believe there should at least be an auto option to change the cursor shape depending on the vim mode (i.e. block in normal mode, line in insert mode and underline in replace mode). Since I believe that modalkit doesn't expose the current mode, it would make more sense to implement this in modalkit.