Add FORM language to allow for overrides

[faquinonez]

Can not override to form language
in vim according to the syntax file definitions

Please take a look on example

I think maybe the error is coming from here
i.e. the extension .frm is already assigned to VB or VBA.

Note: This ticket is related but no the same as the open issue #2418.

[Alhadis]

Linguist doesn't have a language called "FORM" listed, parcero. =)

Only languages that are listed in languages.yml can be selected for highlighting.

[lildude]

If you’ve got the bandwidth, we’d happily accept a PR implementing this.