Support case insensitive purposes

[jesstelford]

Problem Description

Currently, purposes must have a particular case to be detected / valid, which differs depending on the source:

@markings/source-comments

Requires purposes to be in uppercase to be detected. Anything else will be silently ignored.

✅ Works:

// TODO: Do a thing

❌ Silently ignored:

// todo: Do a thing
// Todo: Do a thing

@markings/source-react

Requires purposes to be in lowercase, else will throw an error.

✅ Works:

<Marking purpose="todo" description="Do a thing" />

❌ Throws an error:

<Marking purpose="Todo" description="Do a thing" />
<Marking purpose="TODO" description="Do a thing" />

Error: Purpose must be one of question, todo, fixme

Proposal

To reduce user errors and confusion (when comment sources are silently ignored), I propose:

1. Make all purposes case insensitive:

   // Todo: Do a thing
   <Marking purpose="TODO" description="Do a thing" />

2. Normalise the output of all purposes to lowercase (so TODO becomes todo, and Fixme becomes fixme, etc)