refactor: minor code improvements

[jobveldhuis]

Thanks for this plugin, @LuudJanssen. In this PR, I have made some minor code adjustments to improve readability and increase the semantic value of the code. These changes can roughly be split into three different subjects, which I will outline, below:

Swap returning false for returning null when no result can be returned

Some functions return either a result or false when no result can be provided. I do not think this is semantically correct, as I expect false to be returned only when in a type guard or another assertion function. When a function parses and returns data, I expect either an error to be raised or null to be returned as a value to signal the parsing was completed successfully, but no valid result could be retrieved from parsing, therefore returning null.

Change is-github-alert-declaration.ts to parse-github-alert-declaration.ts

The function contained in this file was called parseGithubAlertDeclaration. I believe file names should follow function naming and have changed this in this PR.

Add type guard function to check whether provided type is a GitHub alert type

Before this PR, we needed to typecast the type variable in parseGithubAlertDeclaration twice. Once to check whether the type was correct and twice when returning. I have added a utility to check whether a type is a GitHub alert type, which borrows its logic directly from the previous code. The second advantage of stripping this logic into a separate function is that we can test that function directly and ensuring that, during parsing, only the correct types are being parsed - without having to test the flow of multiple functions.

[LuudJanssen]

Thanks @jobveldhuis, LGTM!

[LuudJanssen]

Released in v1.0.5.

@jobveldhuis minor comment, but your commit prefix should not start with feat if it does not actually add any functionality. refactor would have been most appropriate here I think. (It tripped up auto-versioning based on commit messages.)