Lint pass keyboard_svg

[narglab]

I ran the Biome linter on keyboard_svg.js and did some cleaning.
before - 476 errors, 38 warnings, 90 infos
after - 28 warnings

Here are the main changes:
• declare variables once, before use, and in outer block
• use triple equals for comparisons
• remove a few unused parameters
• remove one duplicate else-if clause
• added parameter radix=10 to parseInt()

Strings now use templates for style,
I think this is more legible this way
but feel free to disagree and I'll revert to concatenation.

For some reason,the linter still flags most functions as unused,
although they're definitely called.

Finally I formatted comments with author names,
used block comments where appropriate,
and replaced series of slashes with equal signs,
to make browsing comments easier.

[narglab]

One commit mentions "global_y" but I did not rename it in the end,
because the "const y_" variables avoid redefinition already.