feat: more anime combinators

Animations like the ones suggested in #132 should not need to be hard-coded into our spinner library, since they just involve translating a static string across an empty space. Such animation frames can easily be generated with something like this:

```lua
-- UNTESTED CODE btw
local function bouncer(icon, width)
local frames = {}
local padding = width - vim.fn.strdisplaywidth(icon)
assert(padding > 0)
for i=0, padding do
table.insert(frames, string.format("%s%s%s", string.rep(" ", i), icon, string.rep(" ", width - i))
end
for i=1, padding do
table.insert(frames, string.format("%s%s%s", string.rep(" ", width - i), icon, string.rep(" ", width))
end
return frames
end
```

This kind of thing can and should be added to `spinner` as a combinator so that users can roll their own anime

Other combinator ideas (resurrect HTML tags with vengeance):
- [ ] ``-style scrolling text
- [ ] ``-style blinkers

Fidget could even support animated epic gamer highlights (though this will require supporting `_style` fields as functions):
- [ ] "breathing" highlights that fade brighter and darker 😎
- [ ] rainbow highlights 🌈

Start by inspecting the existing spinner combinator API and how animation frames are represented. Use the proposed bouncer example as the first scope, then review whether marquee- and blink-style combinators fit the same interface. Done means users can compose these animations without hard-coding every frame, with coverage for the supported behavior.

Written by the indexing model from the issue text.