Add Identifier wrapper that strips backticks from token text

[adammcarter]

Motivation

Following the issue opened in https://github.com/apple/swift-syntax/issues/1936, take the code below as an example:

let x = y.`z`()

When wanting to pull out the character

`z`

you will have to manually strip the backslashes to get the z character.

The backslashes are completely valid in this case but often using the Swift syntax you might want to remove the backslashes to get the sanitised name of the token for your own purposes which can introduce unnecessary boilerplate.

Solution

Thinking forward, this new Identifier type allows an abstraction over tokens to further sanitise/strip data out, for example, backticks (as in this PR) or future updates like:

• combined characters, e.g. é
• # symbols such as in #if
• comment blocks like /*, /// or //

With the changes in this PR we can take the same example code:

let x = y.`z`()

And by calling [token].identifier.name we can get z without the backticks.

Alternatives considered

Stripping the backslashes when returning the .text is one solution, but the backticks are a valid part of the text and so wouldn't be an accurate way of returning the source code

Trimming the backticks as part of trimmedDescription is a valid and simpler solution IMO, but doesn't allow for future implementations like the above under the Identifier abstraction