fix(theme): Replace non-palette color "magenta" with palette color "red" in monokai_pro_spectrum

[LeoniePhiline]

Fixes #5249

1f0a76f

"magenta" is not defined as hex code in the theme's palette and thus depends on the terminal emulator color theme.

This commit replaces the color with "red", which is defined.

a6eb461

After 1f0a76f, a remaining issue mentioned in #5249 is:

Arguably, yellow on magenta has low contrast by definition (even if it was hex-coded) and thus is bound to be more or less hard to read.

The root cause is that unlike others, this theme uses red as color for builtin types, keywords and operators. Therefore, red is not a good error color. (See #2433)

Note that #2433 should no longer be a problem, since #5419 was merged.

The second uses red + bold for "error", like a range of other themes:
ayu_dark, onelight, solarized_light, solarized_dark, onedarker, heisenberg, ayu_mirage, hex_steel, ayu_light, onedark

Example rust-analyzer error: