[Bug Report] el-time-picker value-format 'time=MM:hh:ss' Wrong selection of input part of input

### Element UI version
2.15.5

### OS/Browsers version
win10

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/sroke/pen/dyRybZV

### Steps to reproduce
if format = "时间是: HH:mm:ss"
When the mouse is placed on the HH of the drop-down box, the content of the input box is not in the HH part, but on the first two characters（时间）

### What is Expected?
Calculate the offset according to format and locate the position automatically selected in the input box

### What is actually happening?
···
this.$emit('select-range',0,2)
this.$emit('select-range',3,4)
this.$emit('select-range',6,8)
···
It's fixed