Default to using a diff when tracking source dates

[worksofliam]

Changes

Removes the ability to configure which mode will be used to track source date changes, and we now default to diff, which tracks changes based on the previous save of the file.

No functional changes here, only UX changes. Documentation may need to be updated after merge.

How to test this PR

Examples:

1. Enable source dates, gutter optional but nice to have on
2. Make changes to a file (optionally see the gutter update)
3. Save the file and ensure all the source dates aren't set to today and only the changed lines are updated.

Checklist

• have tested my change
• have created one or more test cases
• updated relevant documentation
• Remove any/all console.logs I added
• have added myself to the contributors' list in CONTRIBUTING.md

[github-actions]

👋 A new build is available for this PR based on 1b6a205.

• Download here.
• Read more about how to test

[SanjulaGanepola]

Just a couple comments + we should update the description for code-for-ibmi.showDateSearchButton:

If enabled, will show the 'Date search' button in the status bar when source dates with diff mode is enabled.

[SanjulaGanepola]

Changes look good and it works as expected now!

Going back to my previous comment, shouldn't we drop the "when source dates with diff mode is enabled." for showDateSearchButton?

[worksofliam]

@SanjulaGanepola I am following now. I removed the showDateSearchButton config option.

[SanjulaGanepola]

I was thinking just drop that part of the description, but your completely right in that we don't need that option all together. Looks good!