Fix offset conversions at the end of the document

[pkukielka]

Fixes #2196

Changes

Conversion of the end of range to offset was buggy.
VSC sometimes sends -1 as line index. If the start or end position of the range is outside the document, the corresponding offset should be set to 0 or the document's text length, respectively.
But in case of the end range we were setting it to 0.

To avoid that mistake:

• I changed Position::toOffset method name to Position::toOffsetOrZero to be very explicit of what it does. It should not be used for ranges, but sometimes we only have a single position (like for a text insert).
• I added Range::toOffsetRange which returns pair of offsets, correcting them appropriately depending if position is a start or end

Test plan

1. Login to Cody with Free/Pro user
2. Create new empty python file called quicksort.py, make sure it's empty
3. Ask Cody to "create a quicksort function"
4. Click on Apply, make sure code is added to source code file
5. Click on Reject

Edit should be properly rejected and file should be empty again.

[mkondratek]

lgtm