feat(fit-preview): optionally double comp_length when showing preview

[jimeh]

Note: This was a 5 minute hack just to see if I could fix it, so more than happy to hear any feedback and change things as needed :)

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When previews are enabled, they typically take up half the width of the popup. This adds an option to make it double the width when preview is enabled, hence ensuring that the full text of completion items are visible.

Screenshots

Before

With preview:

Without preview:

After

With preview:

Without preview: