fix(vim.text): handle very long strings (#30075)

Lua's string.byte has a maximum (undocumented) allowable length, so
vim.text.hencode fails on large strings with the error "string slice too
long".

Instead of converting the string to an array of bytes up front, convert
each character to a byte one at a time.

test/functional/lua/text_spec.lua

@@ -20,5 +20,11 @@ describe('vim.text', function()
         eq(input, vim.text.hexdecode(output))
       end
     end)
+
+    it('works with very large strings', function()
+      local input, output = string.rep('😂', 2 ^ 16), string.rep('F09F9882', 2 ^ 16)
+      eq(output, vim.text.hexencode(input))
+      eq(input, vim.text.hexdecode(output))
+    end)
   end)
 end)