Fix builtin.registers

[mathiasuk]

Fix editing a letter register (A-Z) would always append to the register, fixed by editing a-z instead.

Fix picker not updating after editing a register.

Fix editing of read-only registers

Description

Fix three small bugs reported in #3462:

• Editing read-only registers results in error
• Editing named registers appends to content instead of replace
• Editing a register does not refresh the picker.

Fixes #3462

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list relevant details about your configuration

• [ x] Edited named and read-only registers, named registers are updated as expected, read-only are ignored, picker is refrshed.

Configuration:

• Neovim version (nvim --version): NVIM v0.11.0
  Build type: Release
  LuaJIT 2.1.1741730670
• Operating system and version: Debian Testing 13

Checklist:

• [ x] My code follows the style guidelines of this project (stylua)
• [ x] I have performed a self-review of my own code
• [ x] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations) ((Not applicable)