Add --rc flag to changelog script

[Gudahtt]

The changelog script now accepts an --rc flag to tell it whether to add new changes to Unreleased or to the header for the current version.

Previously this was inferred from whether the current version matched the most recent tag. However this method only works for the first update. Using a flag simplifies this logic, and makes it possible to manually re-run this for further updates to a release candidate.

Relates to #10752

Manual testing steps:

To update the changelog with unreleased changes:

• Run yarn update-changelog
• See that it puts changes under the "Unreleased" header

To update the changelog for a release candidate:

• Run yarn update-changelog --rc
• See that it puts changes under the header for the current release candidate

[metamaskbot]

Builds ready [a61c161]

• builds: chrome, firefox, opera
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts

Page Load Metrics (589 ± 62 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	48	86	58	9	4
		domContentLoaded	385	834	588	129	62
		load	386	835	589	129	62
		domInteractive	385	834	587	129	62

[metamaskbot]

Builds ready [87bb68a]

• builds: chrome, firefox, opera
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts

Page Load Metrics (599 ± 53 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	45	78	61	11	5
		domContentLoaded	399	758	597	111	53
		load	401	759	599	111	53
		domInteractive	399	758	597	111	53