Refactoring of merge message class

[Kieranties]

Refactoring of the MergeMessage class to enable simpler (and consistent) approach to adding new regular expressions. This is a prelude to an additional PR where I hope to enable custom patterns through config.

• All patterns are now stored in a collection. The collection is enumerated to apply the order in which they are checked (currently the same order previously used)
• Each pattern has a required SourceBranch capture group
• Pull request patterns have a required PullRequestNumber capture group
• All patterns (except Bit Bucket pull request) have an optional TargetBranch capture group. TargetBranch is required for Bit Bucket pull requests

This introduces the following breaking changes:

Merge message	Previous result	Breaking Change
Merge pull request # from feature/one	Pull Request Number = 0 Merged Branch = feature/one	No match
Merge pull request # in feature/one from feature/two to master	Pull Request Number = 0 Merged Branch = feature/two Target Branch = master	No match
Merge pull request #1234 from feature/one into dev	Pull Request Number = 1234 Merged Branch = feature/one into dev Target Branch = dev	Pull Request Number = 1234 Merged Branch = feature/one Target Branch = dev
Merge pull request #1234 from feature/one from feature/two to master	Pull Request Number = 1234 Merged Branch = feature/two Target Branch = null	Pull Request Number = 1234 Merged Branch = feature/two Target Branch = master
Finish V4.0.0 into master	Merged Branch = V4.0.0 into master Target Branch = master	Merged Branch = V4.0.0 Target Branch = master

[asbjornu]

I think all of the old matches make no sense, so I welcome these breaking changes. I'll wait for others to reply before merging, but this LGTM.

[Kieranties]

@asbjornu I've pre-emptively raised #1488 which introduces the actual feature set I've been working towards. It favours KeyValuePair over the custom class but does have a dependency on the breaking changes in this request.

If you're happy with the breaking changes, should I close this and move the breaking change info into that request?

[asbjornu]

@Kieranties: No, we'll keep this open for a few days before merging so @GitTools/developers can have a chance to review it. If I hear nothing, I'll merge and you can rebase #1488 on master afterwards.

[asbjornu]

@Kieranties: Since we've now decided to bump the major version due to other breaking changes, I would highly appreciate if you could rebase this PR and fix the merge conflicts. 🙏

[Kieranties]

@asbjornu I'm tied up for a few days, but will get this done. I'll also rebase/update #1488.

[asbjornu]

@Kieranties: Excellent!

[Kieranties]

@asbjornu Rebase complete. Let me know of any issues. If the PR gets completed I'll do the follow up work on #1488. There are some additional tests added to cater for the TFS merge messages.

[Kieranties]

@asbjornu I see this is out of date again. I've updated my branch to chase things up.

[asbjornu]

Thanks, @Kieranties. 🙏

[Kieranties]

Awesome! I'll get #1488 fixed up this evening!