Add workbench.fontAliasing.auto option

[Cryrivers]

Add workbench.fontAliasing.auto option, which applies default on non-retina displays and antialiased on retina displays automatically, adding some pleasure when moving vscode around multiple displays.

[msftclas]

All committers have signed the CLA.

[bpasero]

@Cryrivers seems to work fine but I would make one change: instead of introducing a monaco-font-aliasing-default class name, simply not set any class by default on the body to get the default browser behaviour. It seems to me that e.g. on my macOS, the default is -webkit-font-smoothing: auto and not -webkit-font-smoothing: subpixel-antialiased

[Cryrivers]

@bpasero thanks for the review. i'll make the change accordingly(i.e. removing monaco-font-aliasing-default). btw, i didn't find any information about what -webkit-font-smoothing: auto should behave. However, on my laptop (MacBook Pro 15" 2016) it behaves as same as subpixel-antialiased. If there's any exceptions, do let me know please.

[bpasero]

@Cryrivers thanks!