lex: fix floating-point number lexing

std/jule/lex/lex.jule

@@ -186,23 +186,23 @@ loop:
 }
 
 fn float_fmt_dotfp(&txt: []byte, mut i: int): str {
-    // skip .f
-    i += 2
-
+    i += 2 // skip .f
     ret float_fmt_e(txt, i)
 }
 
 fn float_fmt_dotp(&txt: []byte, mut i: int): str {
-    // skip .
-    i++
-
+    i++ // skip .
     ret float_fmt_e(txt, i)
 }
 
 fn float_num(&txt: []byte, mut i: int): (lit: str) {
     i++ // Skip dot
-    if i >= txt.len || txt[i] == '_' {
-        ret ""
+    if i >= txt.len {
+        ret str(txt)
+    }
+    if txt[i] == '_' {
+        i--
+        ret str(txt[:i])
     }
     for i < txt.len; i++ {
         let b = txt[i]
@@ -277,7 +277,6 @@ fn is_float_fmt_dotnp(&txt: []byte, mut i: int): bool {
     if txt[i] != '.' {
         ret false
     }
-
     i++
 loop:
     for i < txt.len; i++ {