feat: add dollar as valid identifier char

[moisespsena]

This PR will allow the use of the dollar in identifiers, such as JavaScript.

My intention is to create a template language with UGO, and define the dollar prefixed variables will make it more intuitive.

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[ozanh]

Hi @moisespsena, thank you for the PR.
I haven't considered anything about using uGO as a template language.
I can give it a try under the following conditions and questions.

• As this will be an experimental feature, this should be an optional feature and enabled with a build tag.
• I cannot maintain a template library (this is not addressed in this PR), so you should be responsible for the maintenance.
• $ sign was abused by some JS libraries. Is that what we really want? I think Go's template library preferred to use $ for template variables to reduce parser complexity.
• Do you have a plan to create a template parser?

uGO is used in production a lot, and we cannot introduce anything easily, especially syntax changes.

So making it optional until it is fully implemented and production-ready is the only choice for now.

If you want to go ahead, please create an "exp" package in the project workspace and define a constant like const ExperimentalDollar bool which should be true with tag ugo_exp_dollar. Then you should use if statements in scanner and parser to handle $ conditionally using this constant. It is important to make it constant to let the Go compiler trim unused code if tag is not provided.

Additional requirements about the PR:

• Please remove the code added in docs/tutorial. You don't need to show the implementation detail when we can write it as a readable sentence.
• $ sign is supposed to be used less, so while checking each character it should be the last condition.

[ozanh]

Please open a new PR if you'd like to continue.