Insert str action

[HalFrgrd]

Adds support for a new custom action insertString that allows inserting a literal string of characters into the command line buffer.

• New Action (insertString): Introduces KeyEventAction::InsertString(String) to insert arbitrary literal text into the editor.
• Parsing support: Supports parsing string literals enclosed in single or double quotes, and handles escaped characters (e.g. \n, \t, \r, \xHH).
• Autocompletion: Registers tab-completion candidates for insertString(value) when binding keys.
• Deduplication and Conflict Checking: Integrates custom action display formatting so that bindings show up as insertString(value) in help tables and conflict lists.