change(marks): Align dot below stroke of ṃ U+1E43 on all faces

[alerque]

Closes #384.

SILE Code

```sile \begin{document} \neverindent \nofolios \language[main=und] \script[src=packages/grid] \grid[spacing=30pt] % \grid:debug \begin{raggedright} \begin{script} local files = { "LibertinusSerifDisplay-Regular.otf", "LibertinusSerif-Regular.otf", "LibertinusSerif-Semibold.otf", "LibertinusSerif-Bold.otf", "LibertinusSerif-Italic.otf", "LibertinusSerif-SemiboldItalic.otf", "LibertinusSerif-BoldItalic.otf", "LibertinusSans-Regular.otf", "LibertinusSans-Bold.otf", "LibertinusSans-Italic.otf", "LibertinusMath-Regular.otf", "LibertinusMono-Regular.otf", "LibertinusKeyboard-Regular.otf", "LibertinusSerifInitials-Regular.otf", } local loop = function (func, options) options = options or {} for i, font in ipairs(files) do options.size = "30pt" options.filename = "/usr/share/fonts/OTF/"..font func(options, i, font) SILE.call("kern", { width = "2em" }) options.filename = "/home/caleb/projects/libertinus/"..font func(options, i, font) options.size = "10pt" SILE.call("kern", { width = "2em" }) SILE.call("font", options, { "← "..font }) SILE.call("break") end end loop(function(options, i) local str = "ṃ" SILE.call("font", options, { str } ) end) \end{script} \end{raggedright} \end{document} ```

[alerque]

On hold until design choice about mark positions in #387 is resolved.

[alerque]

Unblocking this PR by aligning the combining dot-under glyph vertically in roughly the same place some other existing dot-under glyphs are aligned. Thus this isn't a breaking style change and puts off the larger cleanup operation for its own PR.