[BUG] Invalid identifier allowed as record field name

[bgribaudo]

Expected behavior To align with Query Editor's behavior, in [1.=true], "1." should be rejected as an invalid identifier.

Actual behavior The parser (as tested in VSCode using the Power Query / M Language extension) accepts the above as valid M.

Additional context https://github.com/MicrosoftDocs/powerquery-docs/issues/30 discusses the grammar for a generalized identifier.

[JordanBoltonMN]

Related to issue #269