MD -> LaTeX: Allow figure/table/etc. to be automatically placed

[LecrisUT]

Basically add a global/directive option for placement which would overwrite the placement bracketOpts:

• auto: let LaTeX engine decide where to place
• a specific value like ht: Use that as is (only for directive)
• undefined: Use the current method

https://github.com/executablebooks/mystmd/blob/42af3800de7d83846ce29b5bf4482e254eeb7214/packages/myst-to-tex/src/container.ts#L112-L115

[rowanc1]

Quite like this - thank you! Keep the issues coming. :)