fix(svg.go): don't drop explicit colors when settings a foregroundColorOverride

[danshick]

this change keeps all custom color classes in place when setting a foregroundColorOverride and chooses instead to style all vt10x.DefaultFG colored text uniformly using a text element selector. the DefaultFG color is set to the result of GetColor(vt10x.DefaultFG) which gives ANSI color LightGrey. this is the same as the previous implicit behavior regardless of whether or not that was intended. if a foregroundColorOverride is set, that color is used instead. this better matches the behavior of the backgroundColorOverride which still respects explicit CSI SGR signals while changing the DefaultBG where it is encountered.

[danshick]

looks like i broke a few tests by deviating from the golden fixture. will get that fixed and update.

[danshick]

ok, tests are passing for me now.

[danshick]

to demonstrate more visually, here is an export of a session with no foreground color set, just termsvg export session.cast:

here's the same session exported with termsvg export -t "#FF0000" session.cast:

and finally, here's the same command run with this change:

still not perfect, explicit background color handling is still buggy, but no different than before the change. i think it's preferable to retain the explicitly set colors rather than render them as black because no classes were added to the styles.