Add appearance setting to adjust tab size

[Repiteo]

Closes #15561

Description

• Previously, tab characters on GitHub Desktop were always rendered at 8 spaces in width, in constrast to the site offering several options for tab size. This PR serves to add that functionality; though, instead of a list of integers, it instead utilizes a TextBox. Any input will be converted to a number and, if valid, clamped between 1 and 16. Much like themes, this setting will update the appearance in realtime

• Though not strictly required, I'd highly recommend this be added either alongside or sometime after Render tab characters properly in diffs #17428. The current tab display is broken, with values under 5 being erroneously truncated for the first character in a line. As such, the ability to access lower values is likely to display erroneously as-is, and this kind of update would be far better suited for an environment where the functionality is working as expected Merged!

Screenshots

Example taken using #17428

23-09-30.12-42-53.GitHubDesktop-dev.mp4

Release notes

Notes: [New] Allow customizing tab character width in spaces

[sergiou87]

Thanks @Repiteo ! I was thinking about this when I worked on #17428

I still have to discuss this with the team (adding settings is always controversial, we don't want many), but maybe it'd make more sense to add that setting to this popover:

[sergiou87]

We just talked about this and decided to follow GitHub.com approach (check https://github.com/settings/appearance), so basically what you just did 😄

The main difference is that the default tab size, as with GitHub.com, will be 8.

[Repiteo]

D'oh! I'm so used to a width of 4 that I made that the default without even thinking lmao. Easy fix though!

[Repiteo]

Now that #17428 has been merged, I've rebased the repo so it can be tested as-is

[fabtjar]

I was just about to suggest adding this feature in to match the website settings. Looking forward to using it!