Fixed the panic when type a statement similar to let f = 'f' $ in the nushell

[mengsuenyan]

• this PR should close nushell panics if I type let = if $ #9596
• fixes nushell panics if I type let = if $ #9596
• this PR should close Crash in parser when backspacing over variable name #9826
• fixes Crash in parser when backspacing over variable name #9826

fixed the following bugs:

# type following statements in the nushell
let f = 'f' $;
mut f = 'f' $;
const f = 'f' $;

# then remove variable f, it will panics
let = 'f' $;
mut  = 'f' $;
const = 'f' $;

[sophiajt]

Can you add tests to this to ensure it's working?

[sophiajt]

thanks!