parser: add p.show() to ease debugging parser problems, that need more context

Using it, produces output like this on stdout:
```
>> vlib/v/tests/typeof_type_test.v:4:        keyword `typeof`  token `(`  token `[`      keyword `fn`                      token `(`  name `s`  name `string`
>> vlib/v/tests/typeof_type_test.v:8:        keyword `typeof`  token `(`  token `[`      name `int`                        token `]`  token `)`  token `.`
>> vlib/v/tests/typeof_type_test.v:9:        keyword `typeof`  token `(`  token `[`      name `int`                        token `]`  token `)`  token `.`
>> vlib/v/tests/typeof_type_test.v:13:       keyword `typeof`  token `(`  token `[`      name `u32`                        token `]`  token `)`  token `.`
>> vlib/v/tests/typeof_type_test.v:14:       keyword `typeof`  token `(`  token `[`      name `u32`                        token `]`  token `)`  token `.`
>> vlib/v/tests/typeof_type_test.v:18:       keyword `typeof`  token `(`  token `[`      name `string`                     token `]`  token `)`  token `.`
```

Note: this tracing output is *deliberately on `stdout`*, instead of `stderr`,
so that you can filter it more easily, without saving it to a file first, i.e.
you can use it while developing/debugging parser issues like this:
`./v -o vnew cmd/v && ./vnew some_file.v | grep some_file.v`

-> this will show only the parsing context for the constructs you are debugging,
for that specific file *only*, instead of for all of `builtin` and the imported files etc.

vlib/v/parser/parser.v

@@ -21,6 +21,7 @@ mut:
 	file_base         string       // "hello.v"
 	file_name         string       // "/home/user/hello.v"
 	file_name_dir     string       // "/home/user"
+	file_display_path string       // just "hello.v", when your current folder for the compilation is "/home/user/", otherwise the full path "/home/user/hello.v"
 	unique_prefix     string       // a hash of p.file_name, used for making anon fn generation unique
 	file_backend_mode ast.Language // .c for .c.v|.c.vv|.c.vsh files; .js for .js.v files, .amd64/.rv32/other arches for .amd64.v/.rv32.v/etc. files, .v otherwise.
 	comments_mode     scanner.CommentsMode = .skip_comments
@@ -178,10 +179,15 @@ pub fn (mut p Parser) free_scanner() {
 	}
 }
 
+const normalised_working_folder = (os.real_path(os.getwd()) + os.path_separator).replace('\\',
+	'/')
+
 pub fn (mut p Parser) set_path(path string) {
 	p.file_name = path
 	p.file_base = os.base(path)
 	p.file_name_dir = os.dir(path)
+	p.file_display_path = os.real_path(p.file_name).replace_once(parser.normalised_working_folder,
+		'').replace('\\', '/')
 	p.inside_vlib_file = p.file_name_dir.contains('vlib')
 	p.inside_test_file = p.file_base.ends_with('_test.v') || p.file_base.ends_with('_test.vv')
 		|| p.file_base.all_before_last('.v').all_before_last('.').ends_with('_test')
@@ -4167,3 +4173,23 @@ fn (mut p Parser) trace(fbase string, message string) {
 		println('> p.trace | ${fbase:-10s} | ${message}')
 	}
 }
+
+[params]
+struct ParserShowParams {
+	msg   string
+	reach int = 3
+}
+
+fn (mut p Parser) show(params ParserShowParams) {
+	mut context := []string{}
+	for i in -params.reach .. params.reach + 1 {
+		x := p.peek_token(i).str()
+		if i == 0 {
+			context << '    ${x:-30s}  '
+			continue
+		}
+		context << x
+	}
+	location := '${p.file_display_path}:${p.tok.line_nr}:'
+	println('>> ${location:-40s} ${params.msg} ${context.join('  ')}')
+}