Underscore characters may start an identifier

[FichteFoll]

Via SublimeText/Terraform#43, it was brought to my attention that apparently underscore characters (_ U+005F) are allowed to start an identifier, whereas the current spec does not say so.

hcl/hclsyntax/spec.md

Lines 90 to 111 in 360ae57

	### Identifiers
	Identifiers name entities such as blocks, attributes and expression variables.
	Identifiers are interpreted as per [UAX #31][uax31] Section 2. Specifically,
	their syntax is defined in terms of the `ID_Start` and `ID_Continue`
	character properties as follows:
	```ebnf
	Identifier = ID_Start (ID_Continue | '-')*;
	```
	The Unicode specification provides the normative requirements for identifier
	parsing. Non-normatively, the spirit of this specification is that `ID_Start`
	consists of Unicode letter and certain unambiguous punctuation tokens, while
	`ID_Continue` augments that set with Unicode digits, combining marks, etc.
	The dash character `-` is additionally allowed in identifiers, even though
	that is not part of the unicode `ID_Continue` definition. This is to allow
	attribute names and block type names to contain dashes, although underscores
	as word separators are considered the idiomatic usage.
	[uax31]: http://unicode.org/reports/tr31/ "Unicode Identifier and Pattern Syntax"

As cited, the spec specifically only allows characters in the ID_Start Unicode property, which is defined as:

ID_Start characters are derived from the Unicode General_Category of uppercase letters, lowercase letters, titlecase letters, modifier letters, other letters, letter numbers, plus Other_ID_Start, minus Pattern_Syntax and Pattern_White_Space code points.

In UnicodeSet notation:
[\p{L}\p{Nl}\p{Other_ID_Start}-\p{Pattern_Syntax}-\p{Pattern_White_Space}]

None of these properties include the underscore character. (It is, however, included in ID_Continue through Pc.)

[crw]

Thanks for this report!