refactor: vim.ui.clipboard #26040

Problem:
Platform-specific UI providers should live in `vim.ui.*`. #24164

Solution:
- Move `vim.clipboard.osc52` module to `vim.ui.clipboard.osc52`.
- TODO: move all of `clipboard.vim` to `vim.ui.clipboard`.

ref #25872

runtime/doc/api.txt

@@ -3598,7 +3598,8 @@ nvim_ui_set_option({name}, {value})                     *nvim_ui_set_option()*
         |RPC| only
 
 nvim_ui_term_event({event}, {value})                    *nvim_ui_term_event()*
-    Tells Nvim when a terminal event has occurred.
+    Tells Nvim when a terminal event has occurred: sets |v:termresponse| and
+    fires |TermResponse|.
 
     The following terminal events are supported:
 

runtime/doc/lsp.txt

@@ -1484,7 +1484,7 @@ get({filter})                                       *vim.lsp.inlay_hint.get()*
         vim.lsp.util.apply_text_edits(resolved_hint.textEdits, 0, client.encoding)
 
         location = resolved_hint.label[1].location
-        client.request("textDocument/hover", {
+        client.request('textDocument/hover', {
           textDocument = { uri = location.uri },
           position = location.range.start,
         })

runtime/doc/provider.txt

@@ -269,12 +269,12 @@ into explicitly by setting the following |g:clipboard| definition: >lua
     vim.g.clipboard = {
       name = 'OSC 52',
       copy = {
-        ['+'] = require('vim.clipboard.osc52').copy,
-        ['*'] = require('vim.clipboard.osc52').copy,
+        ['+'] = require('vim.ui.clipboard.osc52').copy,
+        ['*'] = require('vim.ui.clipboard.osc52').copy,
       },
       paste = {
-        ['+'] = require('vim.clipboard.osc52').paste,
-        ['*'] = require('vim.clipboard.osc52').paste,
+        ['+'] = require('vim.ui.clipboard.osc52').paste,
+        ['*'] = require('vim.ui.clipboard.osc52').paste,
       },
     }
 <

src/nvim/api/ui.c

@@ -512,7 +512,7 @@ void nvim_ui_pum_set_bounds(uint64_t channel_id, Float width, Float height, Floa
   ui->pum_pos = true;
 }
 
-/// Tells Nvim when a terminal event has occurred.
+/// Tells Nvim when a terminal event has occurred: sets |v:termresponse| and fires |TermResponse|.
 ///
 /// The following terminal events are supported:
 ///