Implement canvas measure strategy

[Tyriar]

I have seen some reports like this:

I had a debug session with someone and there were some interesting findings:

• It happened with monospace, Courier New, Consolas, Cascasia Code but not Arial (at least it wasn't obvious)
• Switching fontSize (forcing the char atlas to get refreshed) did not fix the issue
• Increasing the letter spacing mostly worked around the issue

Still not entirely sure why it's happening, but I believe handling this TODO will probably solve the issue and I can get it retested after we've done it:

xterm.js/src/browser/services/CharSizeService.ts

Line 54 in 353a8c5

// TODO: For supporting browsers we should also provide a CanvasCharDimensionsProvider that uses ctx.measureText

[anarchodin]

I get that display (Firefox on Linux, and it's very consistent), but if I use the mouse to get a highlight:

Remove the highlight and it goes right back to the original look. So whatever causes this, it changes when the highlighting is applied.