Generate signature files for lex files.

[nojaf]

I would like to have signature files for *.fsl files as well.

See the example at https://github.com/dotnet/fsharp/pull/15182.

I've made the following changes:

• Made a slight grammar change to FsLex to accept LPAREN IDENT COLON IDENT RPAREN as a function parameter. Because of this, people can specify a type (assuming it is a simple one) that we can use in the signature file.
• Generating a signature file in fslex, similar to how it is done in fsyacc.
• Allow a list of opens to be passed as cli arguments (same in fsyacc).

The assumption now is that each rule does return a token type, which does seem to be the case in the compiler.

[dsyme]

The assumption now is that each rule does return a token type, which does seem to be the case in the compiler.

Could you clarify this please, e.g. link to the relevant line where this assumption is made? It's not always necessaryily the case that there's a token type when using fslex.

[nojaf]

@dsyme right now every *.fslreturns a token type (defined in the *.fsy counterpart).

Example: https://github.com/dotnet/fsharp/blob/97651b3456a7fa23369e189e1365cb38490e2d23/src/Compiler/lex.fsl#L345-L359

Alternatively, an actual return type should be allowed for a rule as well in the grammar.

[dsyme]

@dsyme right now every .fslreturns a token type (defined in the .fsy counterpart).

OK, well, I guess it's OK :)