[fea spec] Allow BCP-47 language tags as language IDs

[moyogo]

One may want use a BCP-47 language-tag record for a name record language ID.

For example instead of the language ID below 0x8000 defined in the Windows or Mac platforms, the name table format 1 allows for BCP-47 language-tag records.

The current AFDKO OpenType Feature File Specification only allows ID numbers in the range in the range 0-65535.

afdko/docs/OpenTypeFeatureFileSpecification.md

Lines 3378 to 3383 in 341c4fd

	An optional `<string attribute>` is one or three space delimited numbers that
	specify the platform, platform-specific, and language IDs to be stored in the
	name record of the name table. If only one number is specified it represents the
	platform ID. The platform ID may be either 1 or 3, corresponding to the
	Macintosh or Microsoft (hereafter called Windows) platforms, respectively. The
	other ID numbers must be in the range 0-65535 but are not otherwise validated.

To allow BCP-47 lang-tag records, BCP-47 strings should allowed as well for the language ID.

For example

feature ss01 {
  featureNames {
    name "Centre diacritics on stem."
    name 3 1 en-US "Center diacritics on stem."
  };
} ss01;