feat: allow starting linenumbers from set value

[akoreman]

Description

For some use-cases, users might want to use code-view to display a snippet of a bigger file (e.g. in a documentation website for a software library or in something like a diff-view). For this use-case, it can be confusing if the line numbers shown in code-view don't match the corresponding line numbers in the bigger file.

This change adds a lineNumbersStart property to the API to set the number the line numbers start counting from. A different option to add this to the API could be to allow lineNumbers to be either a boolean or a number and, when set to a number, have the value of lineNumbers be the value that sets the starting point of the line numbers.

This feature is similar to the data-start attribute in Prism.js, users migrating from prism to code-view might expect this behavior to be possible.

Visual regression tests in the PR actions are failing because this PR makes visual changes.

Related links, issue #, if available: n/a

How has this been tested?

• unit tests are added for the new behavior
• examples are added to the line numbers test page

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d0f7309) to head (a8b082e).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #19   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           27        27           
  Lines          318       319    +1     
  Branches        30        30           
=========================================
+ Hits           318       319    +1     

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