docs(lua): vim.str_utf_{start,end,pos} #24424

Closes #24422

runtime/lua/vim/_meta/builtin.lua

@@ -118,6 +118,53 @@ function vim.stricmp(a, b) end
 --- @param use_utf16? any
 function vim.str_byteindex(str, index, use_utf16) end
 
+--- Gets a list of the starting byte positions of each UTF-8 codepoint in the given string.
+---
+--- Embedded NUL bytes are treated as terminating the string.
+--- @param str string
+--- @return table
+function vim.str_utf_pos(str) end
+
+--- Gets the distance (in bytes) from the starting byte of the codepoint (character) that {index}
+--- points to.
+---
+--- The result can be added to {index} to get the starting byte of a character.
+---
+--- Examples:
+--- <pre>lua
+--- -- The character 'æ' are equal to the bytes '\xc3\xa6' (using UTF-8)
+---
+--- -- Returns 0 because the index is pointing at the first byte of a character
+--- vim.str_utf_start('æ', 1)
+---
+--- -- Returns -1 because the index is pointing at the second byte of a character
+--- vim.str_utf_start('æ', 2)
+--- </pre>
+---
+--- @param str string
+--- @param index number
+--- @return number
+function vim.str_utf_start(str, index) end
+
+--- Gets the distance (in bytes) from the last byte of the codepoint (character) that {index} points
+--- to.
+---
+--- Examples:
+--- <pre>lua
+--- -- The character 'æ' are equal to the bytes '\xc3\xa6' (using UTF-8)
+---
+--- -- Returns 0 because the index is pointing at the last byte of a character
+--- vim.str_utf_end('æ', 2)
+---
+--- -- Returns 1 because the index is pointing at the second last byte of a character
+--- vim.str_utf_end('æ', 1)
+--- </pre>
+---
+--- @param str string
+--- @param index number
+--- @return number
+function vim.str_utf_end(str, index) end
+
 --- Convert byte index to UTF-32 and UTF-16 indices. If {index} is not
 --- supplied, the length of the string is used. All indices are zero-based.
 ---