~ character in the package name breaks generated code

[rnestertsov]

I've noticed strange behavior with the code generator for my grammar.

I have a "~" char in the package name I define inside my "tm" file and as result code produced by textmapper becomes invalid.

Example:
Package name inside tm file:

package = "git.sr.ht/~rn/lang"

Generated lexer.go (the problem exist in other files too):

import (
	"git.sr.ht/~rn/lang"
	"strings"
	"unicode/utf8"
)

// generated by Textmapper; DO NOT EDIT

package git.sr.ht/~rn/lang

Textmapper generate valid code once I remove "~" char from package name.