Fix infinite loop in tokenizer for identifiers starting with underscore

[Copilot]

The tokenizer enters an infinite loop when encountering identifiers starting with underscore (e.g., _0b1), exhausting memory.

Root Cause

read_identifier() regex pattern [a-zA-Z][a-zA-Z0-9_-]* requires first character to be a letter, but tokenizer matches _ as identifier start. Pattern mismatch returns empty string without advancing position, creating infinite Identifier("") tokens.

Changes

• tokenize.mbt: Updated regex to [a-zA-Z0-9_-]+ to match any valid TOML bare key character per spec
• lexer_bug_test.mbt: Added tokenization tests for identifiers starting with underscore
• parser_test.mbt: Added regression test verifying proper error handling

Behavior

// Before: infinite loop, memory exhaustion
@toml.parse("key = _0b1")

// After: tokenizes correctly, fails with "Expected value" error (identifiers not valid as values)
@toml.parse("_key = \"value\"")  // Works as key

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• cli.moonbitlang.com
  • Triggering command: /usr/bin/curl curl -fsSL REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>bug: parse does not terminate</issue_title>
<issue_description>The following test case causes the parsing process to never terminate, and at the end exhausts all memory resources.

///|
test "bad: does not terminate" {
  let toml = @toml.parse("leading-us-bin = _0b1")
  @json.inspect(toml)
}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes bug: parse does not terminate #54

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