feat: Picker titles

[nik-rev]

Pickers now have titles, making it easier to tell what the picker does at a glance

It also gives us flexibility, for when we want to add additional contextual information into the picker which cannot be shown with the columns.

For instance, #11285 would benefit from displaying the current directory of the file picker.

picker type	title
thread_picker	-
dap_launch	-
dap_switch_stack_frame	-
diag_picker	Diagnostics
symbol_picker	Document Symbols
lsp_workspace_command	LSP Commands
workspace_symbol_picker	Workspace Symbols
goto_reference	Goto Reference
goto_single_impl	Goto Implementation
file_picker	Files
command_palette	Commands
changed_file_picker	Changed Files
jumplist_picker	Jump List
buffer_picker	Buffers
global_search	Global Search

Closes #10186

[nik-rev]

reverted

~Note: I made it so that the titles are in the center instead of the left. ~

To do this I made a TitlePosition::{Left,Center} enum. This is how it looks like:

[darshanCommits]

personally I think the left looked better, but okay not anything major.

[David-Else]

I also think the left looked much better, thanks for this great PR!

[nik-rev]

Ok, since 3 of you said you prefer the old look, I'll revert it!