feat: add make fmt for *.sh *.bash

1. add .editorconfig file, add rules for .sh .bash files.
2. add make fmt target, use:
    - gofmt *.go.
    - shfmt *.sh *.bash, shell/completion.bash, shell/key-bindings.bash
        need a left indent due to an outermost if block.
3. add shfmt check for bash scripts in make lint target.
4. install shfmt in actions.

.editorconfig

@@ -0,0 +1,20 @@
+root = true
+
+[*.{sh,bash}]
+indent_style       = space
+indent_size        = 2
+simplify           = true
+binary_next_line   = true
+switch_case_indent = true
+space_redirects    = true
+function_next_line = false
+
+# also bash scripts.
+[{install,uninstall,bin/fzf-preview.sh,bin/fzf-tmux}]
+indent_style       = space
+indent_size        = 2
+simplify           = true
+binary_next_line   = true
+switch_case_indent = true
+space_redirects    = true
+function_next_line = false