Fix: check allowed chars in entrypoints

Motivation and Context

Daniel reported a bug in a contract which contained a Tezos.emit "%here ye people of Tezos" (). This error did not signal it was an invalid entrypoint annotation, and instead failed without giving information.

Description

This MR adds a check for entrypoints.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Example file

$ cat emit_bad_tag.mligo 
let main (_,_ : unit * string ) : operation list * string =
  [Tezos.emit "%hello world" 12], "bye"

Before

$ ligo compile contract emit_bad_tag.mligo 
An internal error ocurred. Please, contact the developers.
Michelson_v1_printer.unparse.

After

$ ligo compile contract emit_bad_tag.mligo 
File "emit_bad_tag.mligo", line 2, characters 3-31:
  1 | let main (_,_ : unit * string ) : operation list * string =
  2 |   [Tezos.emit "%hello world" 12], "bye"

Invalid entrypoint "%hello world". One of the following patterns is expected:
* "%bar" is expected for entrypoint "Bar"
* "%default" when no entrypoint is used.
Valid characters in annotation: ('a' .. 'z' | 'A' .. 'Z' | '_' | '.' | '%' | '@' | '0' .. '9').

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).