rename compiler css option

[benmccann]

Describe the problem

I've never liked the css: boolean option name because it is unclear

Describe the proposed solution

perhaps it could be something like css: 'rendered-inline' | 'external' or externalizeCss: boolean

Alternatives considered

status quo

Importance

nice to have

[stalkerg]

I will be preferred more clear way:

cssUsage: 'rendered-inline' | 'external'

just "CSS" is not enough (not clear for me) but externalizeCss: boolean can be too limited in the future.

[Prinzhorn]

Consider making that 'inline' | 'external' | 'none' then. I want to compile client bundles without any CSS whatsoever. EMH333/esbuild-svelte#109