Add correct parenthesis expression support for command templates grammar

[SegFaulti4]

Current implementation of ANTLR4 grammar for command templates is simplistic and can't recognize some strings, such as:

• {{
• {{}
• {}}{{}

All of listed strings should be recognized as template_word, but currently they lead to token recognition errors, for the latter example the output will be:
line 1:5 token recognition error at: '}' line 1:6 no viable alternative at input '{{'

Even though currently there doesn't seem to be any viable example of shell command that fails to be matched because of this behavior, it still may happen in the future.