Add extension property to SemanticDB

[tanishiking]

Currently, we don't have a way to tell whether a method is an extension method or not.

Proposing adding a new property EXTENSION like this

scalameta/semanticdb/semanticdb/semanticdb.proto

Lines 265 to 289 in b97a504

	enum Property {
	UNKNOWN_PROPERTY = 0;
	reserved 0x1;
	reserved 0x2;
	ABSTRACT = 0x4;
	FINAL = 0x8;
	SEALED = 0x10;
	IMPLICIT = 0x20;
	LAZY = 0x40;
	CASE = 0x80;
	COVARIANT = 0x100;
	CONTRAVARIANT = 0x200;
	VAL = 0x400;
	VAR = 0x800;
	STATIC = 0x1000;
	PRIMARY = 0x2000;
	ENUM = 0x4000;
	DEFAULT = 0x8000;
	GIVEN = 0x10000;
	INLINE = 0x20000;
	OPEN = 0x40000;
	TRANSPARENT = 0x80000;
	INFIX = 0x100000;
	OPAQUE = 0x200000;
	}

     TRANSPARENT = 0x80000;
     INFIX = 0x100000;
     OPAQUE = 0x200000;
+    EXTENSION = 0x400000;

when we have

extension (num: Int)
  def foo = ???

The SymbolInformation for foo will have an EXTENSION property, and kind = METHOD.

[kitbellew]

@tanishiking it looks like a good idea. are you looking for an opinion from others (i doubt you need help with implementation 🙂)?